Move gh-pages into main branch

[Bellangelo]

Closes #3717

What I have done:

1. Go to gh-pages and move all files under the docs folder which Github supports.
2. Go to main branch and run git merge gh-pages --allow-unrelated-histories. This merged the 2 branches even if they had unrelated histories.
3. Updated scripts and references. There are some references in the spec.js files but I think they are unused.

What we need to do after merge:

1. Go to Settings>Pages and under the Branch section select the main branch and the docs folder`.
   
2. After you update the settings, an action will run to deploy the site again.
3. We keep the gh-pages for a few weeks, just to be able to quickly change in case of a bug in the process, and then we delete it.

How to review
To quickly skip and verify the updated files, just follow the What I have done section and then you can just git diff to distinguished what files have been moved and what files contain actual changes.

[Bellangelo]

I don't have the permissions needed. I see the source, but not the branch picker.

How can we test this before we do it? Did you try it out on o a public fork, or who can do that? I expected the scripts/ directory (currently only exists in main) to need some changes to get the outputs to build in the target folder (we must currently have some "release" step for this, I'm not sure how that works) - did we check that?

@lornajane My bad. My fork is the https://github.com/Bellangelo/OpenAPI-Specification which deploys to the https://bellangelo.github.io/OpenAPI-Specification/. I have made some extra changes to the deploy / build scripts based on some finding from @ralfhandl . If you check this action https://github.com/Bellangelo/OpenAPI-Specification/actions/runs/9375129267 you can see that it passes.

The way I have been testing the changes in the site so far is mainly using the fork. The fork copies also some repo settings about the deployment but you have to fork through the Github's UI. If you do this, you will be able to deploy the same way we deploy here but in our own url.

[ralfhandl]

Tested on a fork, respec workflow and github pages work fine

[frankkilcommins]

I've created issue OAI/Arazzo-Specification#207 on the Arazzo repo to change our respec job so that it will work with the new structure.

[frankkilcommins]

Also note that #3900 will need to be incorporated into this PR if it's merged prior to this PR.

[lornajane]

@Bellangelo just wanted to drop you a note to say we haven't forgotten about your excellent work, just a lot of moving parts. We will figure it out.

[lornajane]

Sorry, closed the wrong PR!

[ralfhandl]

TDC decision: @ralfhandl to copy relevant instructions to #3717 and then close this PR.

[handrews]

I got impatient and copied the info over so we could reach zero open PRs 😁