Remove github edit link in non-trunk versions of docs

[SmileyChris]

Description

In the sphinx configuration, remove github edit link in non-trunk versions of docs.

This only triggers specifically for read the docs, since it's based one of their custom environment variables.

Fixes #505

[adiroiban]

Hi Chris.

It looks like this does not work as expected.

We don't have the edit links for the "latest" version - https://towncrier.readthedocs.io/en/latest/tutorial.html

The edit link is present on "trunk" https://towncrier.readthedocs.io/en/trunk/tutorial.html

but I don't think that people are using the "trunk" url.

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The defaul version is "stable" ...aka latest tag - https://towncrier.readthedocs.io/en/stable/

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Note that on Read The Docs, we have 2 builds for each merge in trunk

https://readthedocs.org/projects/towncrier/builds/

The "trunk" build and the "latest" build.

@SmileyChris can you please send another PR that will also include "latest" in the list version with edit enabled ?

Thanks!