sphinx_rtd_theme/layout: Set url_root properly on index, don't use '#'

[rkdarst]

• You can see a practical demonstration of the problem, fully seeing
  the effect if it is not fixed, here (though that example uses the
  alabaster theme, the effect is the same here):
  copy button <img> doesn't work with dirhtml builder, only on root page executablebooks/sphinx-copybutton#110
• This is a copy of a fix from Sphinx. The sphinx pull request is
  sphinx/themes/basic/layout: Set url_root properly on index, don't use '#' sphinx-doc/sphinx#8524

Detailed description:

• url_root is set to # on the index page, which layout.html tries to
  change back to '' (the empty string).
• But, this updated url_root wasn't used in the actual location, as an
  argument to documentation_options.js.
• Thus, clever enough templates, which tried to use
  $DOCUMENTATION_OPTIONS.URL_ROOT inside javascript would fail.
  This was manifested as broken links, which led to this issue:
  copy button <img> doesn't work with dirhtml builder, only on root page executablebooks/sphinx-copybutton#110
• I have eventually traced that back to sphinx itself, and found that
  layout.html tried to fix the problem, but the fixed value wasn't
  used.
• This fix works in my basic test, but I will continue with more tests.
• Review:
  • someone more clever should examine this and make sure it makes
    sense
  • This does not have tests. Should it?

[stsewd]

Thanks, I'm not that familiar with all this, but makes sense. I'll merge after is approved upstream sphinx-doc/sphinx#8524

[rkdarst]

Merged upstream now.

[rkdarst]

thanks!