Edit on Github button errors with 404

[ksuprynowicz]

On https://docs.overte.org/en/latest/home.html when pressing Edit on Github leads to not existing page: https://github.com/overte-org/overte-docs-sphinx/blob/master/docs/source/index.rst

[JulianGro]

This is partially fixed, as in every page other than index.html should link to its source file. index.html is an edge case because it is actually a symlink to home.html. This is because ReadTheDocs doesn't allow setting the default document.