Move spec-specific parts out of gh_pages for spec repos

[peterbrightwell]

Currently some files in gh_pages are manually edited for each spec repo, but we don't use PRs for gh_pages (as it's rebuilt automatically by CI). It would be better to have them on a main branch or branches.

These include intro.md, _config.yml and README.md. The latter is currently different for gh_pages branches, but perhaps we could move it elsewhere anyway?

[garethsb]

Good plan. I think only the heading and the amwa-tv.github.io link is specific to the repo in gh-pages/README.md. The link is in the project description, so I don't think we'd lose much if that file was consistent between all repos or elsewhere?

[peterbrightwell]

This will all be "sorted" in the move to GitHub Actions render and upload to https://specs.amwa.tv/. See #19 and also AMWA-TV/is-template#22, which puts intro.md in the dev branches and will allow us to get rid of gh-pages.

[peterbrightwell]

Sorted in the big changes #19 and #21