Warnings when building documentation

[sajith]

There are some warnings about files that are outside doc directory:

$ cd doc; make html
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v2.4.3
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 4 source files that are out of date
updating environment: 1 added, 4 changed, 0 removed
reading sources... [100%] windows                                                                                                                                                                                                                                                                                             
tahoe-lafs/docs/index.rst:10: WARNING: toctree contains reference to nonexisting document '.github/CONTRIBUTING'
tahoe-lafs/docs/index.rst:10: WARNING: toctree contains reference to nonexisting document 'CODE_OF_CONDUCT'
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] windows                                                                                                                                                                                                                                                                                              
generating indices...  genindexdone
writing additional pages...  searchdone
copying static files... ... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 2 warnings.

The HTML pages are in _build/html.

Build finished. The HTML pages are in _build/html.