fix: update-docs.yml github actions condition

[sdarwin]

Hi,

The idea in this PR is for update-docs.yml to make a conditional decision if it's running on the parent repo or not. If it's not then skip building docs.

Explanation:

Currently there are over 700 forks of the terraform-aws-github-runner repository. We're maintaining a git fork and rebasing from upstream on a certain schedule. When rebasing, it always pulls in update-docs.yml and that file rebuilds the docs and commits them. The automatic extra commit is incorrect and must be deleted because we're keeping careful track of all git commits in the history. You might suggest "rebasing" isn't the best idea, and it should be "merging". But "rebasing" keeps a nice linear git history.

If any git repository syncs their fork on a regular basis, it pulls in the docs from upstream, so it's not necessary to have additional commit messages about building docs again.

If a particular fork really chooses to be "independent" and branch off on their own without merging or rebasing from upstream, they can manually modify update-docs.yml to refer to their own repository specifically. In the majority case, where forks are frequently synchronizing with the parent repo, by any method, they will get their docs refreshed during that process. So all cases may be dealt with correctly.

Ultimately, when there are so many git forks, which are often syncing with upstream rather than being permanent separate forks, it can cause problems to "automatically" commit code.