contributing: Add autogenerated documentation to gitignore

[marisn]

As long as we have in-tree build system, our .gitignore has to grow.
Fixes #3521

[neteler]

(please make the PR title compatible with an entry in https://github.com/OSGeo/grass/blob/main/utils/release.yml as the release notes are autogenerated)

[marisn]

(please make the PR title compatible with an entry in https://github.com/OSGeo/grass/blob/main/utils/release.yml as the release notes are autogenerated)

I do not think this is something that should go to release notes.

[neteler]

Ok. Still I have to check line manually and, in this case, delete it from the extracted PR notes.

[echoix]

What about the "infra:" prefix?
Also, I'm not sure that it excludes everything either, except if I build it myself. Do we take the approach of merging this and having to complete afterwards when we figure out if the python docs create more files.

[neteler]

Looks good to me.

(Un)related: the "infra:" keyword needs to be added to https://github.com/OSGeo/grass/blob/main/utils/release.yml