Improve realtime compiler routing for nested index pages

[emmadesilva]

Consider this setup:

_docs/index.md
_pages/about/index.md
_pages/about/contact.md

Which will generate the following structure:

_site/docs/index.html
_site/about/index.html
_site/about/contact.html

A web server will route this as expected:

/docs/ -> _site/docs/index.html
/about/ -> _site/about/index.html
/about/contact/ -> _site/about/contact.html

# And of course:

/docs -> _site/docs/index.html
/about -> _site/about/index.html
/docs/index -> _site/docs/index.html
/about/index -> _site/about/index.html

So this PR makes so that the realtime compiler can serve any arbitrary nested index page.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (f7c6e21) to head (d1b9ec5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master     #1852   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity      1764      1764           
===========================================
  Files            182       182           
  Lines           4712      4712           
===========================================
  Hits            4712      4712           

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