Add two files and one folder from doc building to git ignoring

[gepcel]

After building docs, there are two files and one folder left that shouldn't be committed. Add them to .gitignore.

On branch main
Your branch is ahead of 'origin/main' by 8 commits.
  (use "git push" to publish your local commits)

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        docs/gallery/
        docs/sg_execution_times.rst
        docs/whats_new.rst

nothing added to commit but untracked files present (use "git add" to track)

[cvanelteren]

Thanks!