fix: correctly update the generated available-plugins during deploys.

[iBotPeaches]

The Problem

🔑 We see on the public docs that available plugins are outdated severely. Roughly 4 years.

This is because files that are generated are supposed to be stored in docs/generated/ and then a script during mkdocs build copies those files out of generated into the final resting place.

- mkdocs-simple-hooks:
    hooks:
      on_pre_build: hooks:copy_generated_files

That script copies 2 files and a directory prior to doc build.

def copy_generated_files(*args, **kwargs):
    shutil.copy("docs/generated/available-plugins.md", "docs/plugins/available-plugins.md")
    shutil.copy("docs/generated/actions.md", "docs/actions.md")
    
    actions_dir="docs/actions"
    if os.path.isdir(actions_dir):
        shutil.rmtree(actions_dir)
    shutil.copytree("docs/generated/actions", actions_dir)

So every release we copy a file that has not been updated in years overwriting the one that was freshly updated via the release. So the "Available Plugins" has been frozen in time for 4 years.

This requires a fix in both repositories since the bug occurred when fastlane/fastlane started writing files not in the generated folder.

The Fix

We git-ignore files in the main tree that are built via automation. This will prevent humans from changing files that should not be changed and push us to the automation/generated path.

This PR on fastlane/fastlane side will fix it going forward - fastlane/fastlane#29729

fixes: #1278

[iBotPeaches]

My preview here shows the up to date available plugins for context

new - https://deploy-preview-1287--fastlane-docs-preview.netlify.app/plugins/available-plugins/
old - https://docs.fastlane.tools/plugins/available-plugins/