[#282]:svarga:docs, retire MkDocs — drop config, top-level pages, theme dirs, build/CI targets; Doxygen publishes to gh-pages root

Removes the MkDocs configuration and all Material-theme remnants now
that the Doxygen tree (front page + I/O + TOPICS + COOKBOOK + USABILITY +
REPORTS, with the curated reports tree) is the single documentation source.

Top-level config:
  - mkdocs.yml

docs/ top-level Markdown pages (none referenced from docs/index.md or
the Doxygen INPUT list):
  - architecture.md + architecture-{core,io,types,integration,parallelism}.md
  - compiler.md, cpp-standard-compliance-report.md, design.md
  - examples.md, filtering-pipeline-rework-report.md
  - s3.md, stl.md, thirdparty.md, workflow.md

docs/ subdirs (mkdocs/Material only — confirmed not in Doxyfile INPUT):
  - overrides/  (Material custom_dir, 6 files)
  - assets/     (Material assets, 12 files)
  - styles/     (MkDocs CSS, 8 files)
  - javascripts/ (MkDocs JS, 8 files)
  - icons/      (Material icons, 22 files)
  - dot/        (2 files — Doxygen has its own DOT integration)
  - skills/     (7 .md files)
  - examples/   (5 .md files — Doxygen EXCLUDEs this dir)

Kept (all currently consumed by Doxygen):
  - docs/index.md           — USE_MDFILE_AS_MAINPAGE target
  - docs/aliases.md         — INPUT-listed alias-vocabulary page
  - docs/curated/           — I/O + TOPICS tree
  - docs/reports/           — flat reports tree
  - docs/links/             — Doxyfile @INCLUDE_PATH
  - docs/tags/              — Doxyfile TAGFILES (cppreference cross-refs)
  - docs/pix/               — Doxyfile IMAGE_PATH
  - docs/doxygen/           — build output

CMakeLists.txt: drop find_program(MKDOCS_EXECUTABLE) block + the
'docs' (mkdocs build) and 'docs-serve' (mkdocs serve --livereload)
custom targets. docs-doxygen target preserved.

.github/workflows/docs.yml: drop the entire deploy-mkdocs job (Python
setup, pip install mkdocs+material, mkdocs build, deploy ./site).
The remaining deploy-doxygen job now publishes to the gh-pages
root (destination_dir removed) instead of /api, so the Doxygen site
takes over the URL that MkDocs used to serve.

Net diff: 88 files / -15,593 / +6.  Local rebuild via scripts/docs-build.sh
is clean (0 warnings); every key landing page still resolves
(curated_io_api, curated_topics, reports_index, USABILITY, REPORTS).

Author: steven-varga

.github/workflows/docs.yml

@@ -8,32 +8,10 @@ permissions:
   contents: write
 
 jobs:
-  # MkDocs — conceptual docs, architecture pages, guides
-  deploy-mkdocs:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout repo
-        uses: actions/checkout@v6
-
-      - name: Set up Python
-        uses: actions/setup-python@v6
-        with:
-          python-version: '3.x'
-
-      - name: Install MkDocs + Material theme
-        run: pip install mkdocs mkdocs-material
-
-      - name: Build MkDocs site
-        run: mkdocs build
-
-      - name: Deploy MkDocs to gh-pages root
-        uses: peaceiris/actions-gh-pages@v4
-        with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: ./site
-          keep_files: true
-
-  # Doxygen — API reference generated from C++ headers
+  # Doxygen — full project documentation site (API reference + curated
+  # I/O+TOPICS axes + COOKBOOK + REPORTS, all generated from headers,
+  # examples, and the docs/{index,aliases,curated,reports}/ markdown).
+  # Publishes to the gh-pages root; MkDocs has been retired.
   deploy-doxygen:
     runs-on: ubuntu-latest
     steps:
@@ -55,7 +33,7 @@ jobs:
           # version-deriving git describe in docs-build.sh sees real releases.
           git fetch --tags --depth=0 origin || true
 
-      - name: Build Doxygen API reference (version from git tag, post-processed)
+      - name: Build Doxygen documentation (version from git tag, post-processed)
         run: scripts/docs-build.sh
 
       - name: Summarise Doxygen warnings
@@ -79,10 +57,9 @@ jobs:
           if-no-files-found: ignore
           retention-days: 30
 
-      - name: Deploy Doxygen to gh-pages/api
+      - name: Deploy Doxygen to gh-pages root
         uses: peaceiris/actions-gh-pages@v4
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./docs/doxygen/html
-          destination_dir: api
           keep_files: true

CMakeLists.txt

@@ -453,25 +453,8 @@ string(TOLOWER "${CPACK_PACKAGE_FILE_NAME}" CPACK_PACKAGE_FILE_NAME)
 include(CPack)
 
 # ─── Developer convenience targets ───────────────────────────────────────────
-find_program(MKDOCS_EXECUTABLE mkdocs)
 find_program(DOXYGEN_EXECUTABLE doxygen)
 
-if(MKDOCS_EXECUTABLE)
-    add_custom_target(docs COMMAND ${MKDOCS_EXECUTABLE} build --config-file ${CMAKE_SOURCE_DIR}/mkdocs.yml --site-dir ${CMAKE_BINARY_DIR}/site
-        WORKING_DIRECTORY ${CMAKE_SOURCE_DIR} 
-        COMMENT "Building MkDocs documentation → build/site"
-        VERBATIM
-    )
-    set(MKDOCS_DEV_ADDR "127.0.0.1:8000" CACHE STRING "host:port for mkdocs serve (docs-serve target)")
-    add_custom_target(docs-serve COMMAND ${MKDOCS_EXECUTABLE} serve --livereload
-        --dev-addr ${MKDOCS_DEV_ADDR}
-        --config-file ${CMAKE_SOURCE_DIR}/mkdocs.yml
-        WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
-        COMMENT "Serving MkDocs with live reload at ${MKDOCS_DEV_ADDR} (Ctrl-C to stop)"
-        VERBATIM
-    )
-endif()
-
 if(DOXYGEN_EXECUTABLE)
     add_custom_target(docs-doxygen COMMAND ${DOXYGEN_EXECUTABLE} Doxyfile
         WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}/doxy