Fix to run check-docs-changes worflow in forks

[kumaranvpl]

Description

git-auto-commit-action does support updating fork PRs as specified here - stefanzweifel/git-auto-commit-action#211.

This fix has been taken from going through above issue and reading following links:

1. https://github.com/stefanzweifel/git-auto-commit-action?tab=readme-ov-file#workflow-should-run-in-base-repository
2. https://github.com/stefanzweifel/git-auto-commit-action-demo-app/pull/33/files
3. https://github.com/sunpy/sunpy/blob/cb2674931dc7529a86c8023eefc152b933adf03f/.github/workflows/changelog_bot.yml

@Lancetnik GitHub notifies that using pull_request_target could be potentially unsafe and could expose secrets - https://securitylab.github.com/resources/github-actions-preventing-pwn-requests/. But GitHub also mentions that it is fine to use pull_request_target to update PR with stuff like adding label, adding comment, etc.

I believe for our use case, which is updating docs in PR, we could use pull_request_target in check-docs-changes CI job. Please keep in mind that we shouldn't use pull_request_target anywhere else.

I have observed that FastAPI also uses pull_request_target in its workflows for automation cases like our case.

Finally, this solution works for fork PRs. I have created a PR in which I have added a dummy file with a dummy class - #1709. I created my PR to this current branch(fix-update-api-docs-action) instead of main branch. check-docs-changes ran successfuly https://github.com/airtai/faststream/actions/runs/10487467300/job/29047872884 and added docs changes as new commit - https://github.com/airtai/faststream/pull/1709/commits

Fixes #1705

Type of change

Please delete options that are not relevant.

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a fix or feature that would disrupt existing functionality)
• This change requires a documentation update

Checklist

• My code adheres to the style guidelines of this project (scripts/lint.sh shows no errors)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• Both new and existing unit tests pass successfully on my local environment by running scripts/test-cov.sh
• I have ensured that static analysis tests are passing by running scripts/static-analysis.sh
• I have included code examples to illustrate the modifications