Update ref/relref documentation about optional extensions

[kaushalmodi]

Update the ref/relref documentation with:

• Mention that the extension portion of the document name is optional.
• Once v0.35 is released, also document the feature requested in Make ref and relref point to the "page I mean" hugo#4312.. Now the index.?? portion for page bundles is optional too.

[bep]

I suggest waiting for 0.35 and make a "overhaul" of this.

[Jibec]

I would love to see this included in documentation, this is vital for internal links while and multilingual site, so the link is language agnostic!

[kaushalmodi]

• Clarify that ref and relref can be used to link only regular pages (not section pages, taxonomy pages, etc.) (ref)

[stale]

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[jmooring]

This was resolved on September 5, 2020 with #1213.

Note that these statements were removed from the documentation because they are no longer true:

ref looks up Hugo “Regular Pages” only. It can’t be used for the homepage, section pages, etc.

relref looks up Hugo “Regular Pages” only. It can’t be used for the homepage, section pages, etc.