chore: remove old Hugo docs generation (#856)

* chore: remove old Hugo doc publishing workflows

These workflows pushed the docs.kosli.com/ directory to docs-main and
staging-docs branches for Netlify Hugo builds. No longer needed now that
docs are generated via Mintlify in kosli-dev/docs.

Part of kosli-dev/docs#42

* chore: remove docs-gen job from release workflow, drop Hugo helm-docs output

- Remove the docs-gen job that generated CLI reference markdown and
  pushed docs.kosli.com/ to the docs-main branch on every release
- Remove the docs-gen dependency from the failure-notification job
- Remove the helm-docs line that wrote to docs.kosli.com/content/helm/

The docs-repo-dispatch job is kept — it notifies kosli-dev/docs of new
CLI releases so Mintlify can regenerate.

Part of kosli-dev/docs#42

* chore: remove old docs Makefile targets and scripts

Remove targets: cli-docs, legacy-ref-docs, generate-json-metadata,
hugo, hugo-local, test_docs, check-links, and the second helm-docs
output line that wrote to docs.kosli.com/.

Delete scripts:
- bin/test_docs_cmds.sh (tested commands embedded in Hugo markdown)
- hack/generate-old-versions-docs.sh (generated legacy version docs)

Part of kosli-dev/docs#42

* chore: remove docs.kosli.com/ Hugo site directory

Remove the entire Hugo documentation site (239 tracked files): config,
layouts, content, assets, static files, and Netlify config. Documentation
is now maintained in the kosli-dev/docs Mintlify repo.

Part of kosli-dev/docs#42

* chore: remove Hugo formatter, make Mintlify the default doc format

- Delete internal/docgen/hugo.go and hugo_test.go
- Remove --mintlify flag from `kosli docs` (Mintlify is now the only format)
- Update generate_test.go and formatter_test.go to use MintlifyFormatter
- Update docs_test.go to compare against Mintlify golden files
- Delete Hugo golden test fixtures (testdata/output/docs/hugo/)

Part of kosli-dev/docs#42

* chore: clean up remaining Hugo docs references

- Remove docs.kosli.com/ patterns from .gitignore
- Delete .clear-files (preserved Hugo-generated content during deploys)
- Delete .htmltest.yml (pointed at Hugo build output)
- Update README.md and dev-guide.md docs links
- Update release-guide.md: remove old docs generation/hosting sections,
  document Mintlify dispatch workflow

Closes kosli-dev/docs#42

* chore: mark docs cleanup slices as complete in TODO.md

Author: dangrondahl