I could write an extension for this, but it seems a fairly generically useful feature, with very minimal implementation, so perhaps it can go directly into sphinx, hence this pull request.
The patches contain the following changes:
Fix html_static_path documentation: files are copied to outdir/_static, not outdir.
New html_extra_path option. These files are excluded from the regular build, but are copied at verbatim to outdir. The main use of this is files like robots.txt or .htaccess which you otherwise have to copy by hand after every build. The html_static_path only copies files to _static so does not fit the bill for these sort of files.