Update Sphinx build conf to exclude files not needed for specific formats

[deoren]

While reviewing the build config for the official sphinx-doc project I found this:

epub_exclude_files = ['_static/opensearch.xml', '_static/doctools.js',
                      '_static/jquery.js', '_static/searchtools.js',
                      '_static/underscore.js', '_static/basic.css',
                      'search.html', '_static/websupport.js']

Excluding those files for the epub format makes sense; epub readers are not likely to be executing JavaScript or applying styles intended specifically for the HTML format.

There are probably other optimizations that can also be applied, such as skipping searches of specific directories for source files (e.g., the build directory).

[deoren]

There are several PRs already in line to be reviewed/merged before work can be done on this one (at least without requiring rebase work on the other PRs).