[CI] Reduce repo size by auto-trimming gh-pages branch

[marcdumais-work]

What it does

Use "force_orphan" option of GH action peaceiris/actions-gh-pages@v3

With this change, the API documentation that we generate as part of our CI will overwrite the gh-pages branch, only keeping the latest generated documentation. This will reduce the repository size (when cloned, from close to 3GB to ~150M.

As a consequence, we will lose the history of API documentation. It will still be possible to manually generate it locally (yarn docs).

Fixes #11625

How to test

I pushed the change on my fork of this repo. See resulting gh-pages branch: https://github.com/marcdumais-work/theia/tree/gh-pages.

You can also try cloning my fork, and confirm that about 150M is transferred, instead of ~2.8G:
$ git clone git@github.com:marcdumais-work/theia.git

update: it took a little while, but my repo size, as reported by GitHub, has shrunk to the expected, much smaller size, without further action on my part:

Generated doc (my fork): https://marcdumais-work.github.io/theia/docs/next/index.html

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[marcdumais-work]

I aded an entry to discuss this in the next dev-meeting

[vince-fugnitto]

LGTM 👍

We are only interested in providing documentation for the latest state at the moment, and it is is always possible to re-generate the documentation for any given commit.

The force_orphan option is correct according to the documentation:

We can set the force_orphan: true option. This allows you to make your publish branch with only the latest commit.

I also confirmed that your fork has a much smaller gh-pages size.

[marcdumais-work]

Thanks @vince-fugnitto