Too many files are copied over by docs rule

[jacobshirley]

For example:

bazel run //:mkdocs.serve
INFO: Analyzed target //:mkdocs.serve (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //:mkdocs.serve up-to-date:
  bazel-bin/mkdocs.serve.sh
INFO: Elapsed time: 0.244s, Critical Path: 0.00s
INFO: 1 process: 16 action cache hit, 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/mkdocs.serve.sh
INFO    -  Building documentation...
INFO    -  Cleaning site directory
INFO    -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
             - docgen/docs/defs.md
             - docgen/docs/extensions.md
             - docgen/docs/repositories.md
             - docs/README.md
             - docs/other-info-2.md
             - docs/other-info.md
             - index/README.md
INFO    -  Doc file 'README.md' contains an unrecognized relative link 'e2e/git_last_updated/', it was left as is.
INFO    -  Documentation built in 0.20 seconds
INFO    -  [09:18:09] Serving on http://127.0.0.1:8000/

As can be seen, many files are copied over and are not used:

 - docgen/docs/defs.md
 - docgen/docs/extensions.md
 - docgen/docs/repositories.md
 - docs/README.md
 - docs/other-info-2.md
 - docs/other-info.md
 - index/README.md