docs: Fix literalinclude in the Bazel build

When building the docs with Bazel, literalinclude directives
in the Sphinx docs were not working. This is because the
files-to-include were not part of the docs build graph.
Sphinx-in-Bazel is currently configured to warn about the
missing source files but otherwise proceed with the build.

It's OK if this change has not caught every single missing
include file. Later we will formally verify that the new
Bazel-built site is exactly the same as the old GN-built site.
During that formal verification we will catch the rest of
the broken literalinclude directives.

Bug: 384082025
Change-Id: Icf25320f6ccb1fbd09c0f59a4ec5abe43273144d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/254632
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Kayce Basques <kayce@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Docs-Not-Needed: Kayce Basques <kayce@google.com>