Fix readthedocs to keep the edit link for `latest` in addition to `trunk`

[SmileyChris]

Description

Followon from #596 Let's make it work for the latest version since that's the version more likely to be read...

[adiroiban]

Looks good. Thanks. I can see the edit button on

• https://towncrier.readthedocs.io/en/latest/
• https://towncrier.readthedocs.io/en/trunk/

There is still an edit link on "stable" and previous tags, but this is because there were no new builds.

• https://towncrier.readthedocs.io/en/stable/

Maybe this is a good reason do make a new release