Make gh-rez external link dynamic

[BryceGattis]

Closes #1690. Applies suggested fix from @JeanChristopheMorinPerso.

[BryceGattis]

@JeanChristopheMorinPerso Question though: This change only affects the URL in the PR right? It seems to still point just to 'main' when I do a local Sphinx build. Might be something nice to note as a comment if so.

[JeanChristopheMorinPerso]

@BryceGattis do you think we should use file://... when not building on RTD and if the CI environment is not set? (CI is a commonly set environment variable by most CI systems).

[BryceGattis]

@JeanChristopheMorinPerso

The READTHEDOCS environment variable is only set when we push to GitHub right? Just confirming.

I'm unsure exactly what I would need to put in the config file going down the file://... approach as you mentioned.

[JeanChristopheMorinPerso]

I think we can go with this and change later if needed.