Documentation actions improvements

[Pikachu920]

Description

This PR improves the functionality of the doc actions. It adds the following:

1. Javadoc exclusions as provided by @AyhamAl-Ali
2. Fail gracefully when the SSH key secret is not available
3. Deletes the nightly documentation of a branch when that branch is deleted
4. Support doc templates at /docs/templates instead of /docs-templates (Move documentation into its own folder #5580)
5. Attempt a rebase before committing to reduce chance of race condition
6. Only push nightly documentation if there is actually a change between commits (this should cut down on useless commits to skript-docs)

Example workflows and commits:

1. Javadoc filters: exclusions/workflow/generated javadocs
2. Fail gracefully when SSH key secret is not available: workflow with secret/workflow without secret
3. Deletion of nightly docs upon branch deletion: workflow/commit
4. Support templates at /docs/templates instead of /docs-templates: workflow using docs/templates (ignore the failure to push, it's just cause its using a testing branch on skript-docs. I believe it'll work fine in practice.)
5. Attempt a rebase before commit: workflow/logic
6. Don't push changes if there were no changes to the docs: workflow with changes/workflow without changes

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Target Minecraft Versions: N/A
Requirements: None
Related Issues: None

[TheLimeGlass]

Moving documentation to it's own folder #5580

[Moderocky]

Looks good. Thanks for your work on this.