chore(docs): add build_test and remove dead bazel-manpages target

  Add //docs:man_pages_build_test to catch regressions in the hermetic
  man page build. Remove the now-dead bazel-manpages Makefile target
  which was previously called by Bazel but is no longer needed since
  manpages_impl.py drives the build directly.

Signed-off-by: Jack Luar <39641663+luarss@users.noreply.github.com>

Author: luarss

docs/BUILD.bazel

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright (c) 2026, The OpenROAD Authors
 
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
 load("//bazel:man_pages.bzl", "man_pages")
 
 man_pages(
@@ -66,3 +67,8 @@ man_pages(
     ],
     visibility = ["//visibility:public"],
 )
+
+build_test(
+    name = "man_pages_build_test",
+    targets = [":man_pages"],
+)

docs/Makefile

@@ -124,13 +124,8 @@ $(CAT2_DIR)/%.2: $(CAT2_DIR)/%.md
 $(CAT3_DIR)/%.3: $(CAT3_DIR)/%.md
 	mv $< $@
 
-# Single-invocation target used by //docs:man_pages (Bazel). Chains the
-# preprocessing step (README symlinks + roff-compat conversion) with the
-# cat and web outputs that the Bazel rule copies into its declared outputs.
-bazel-manpages: preprocess cat web
-
 # Phony targets
-.PHONY: all bazel-manpages preprocess
+.PHONY: all preprocess
 
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