Add other workflows and start to update documentation.

[iamsophiesk]

WIP!

• Added extra workflow files.
  • Renamed them all numerically to provide some order (as they need to be in the root directory):
    • 00: Helper workflows.
    • 01: Build tasks.
    • 02: Test tasks.
    • 03: Deploy tasks.
    • 04: none yet, but see these being scan tasks.
  • Renamed the existing composer.yml file to 01-build-composer.json to match the others.
• Added a couple of actions that we use around the place as well.
• Added README file. I hope you all like readmes. I like readmes. This is a hell of a readme.
• Added some example workflows based on the projects we have in our old GitHub repo.
• Added some new workflows that I haven't tested yet, based on some things we have in projects I'm on... cannot guarantee they'll work.
• Added a MIGRATION.md file that I'm hoping to populate as a cheat sheet for using these files. The Access one will be easy - but I might lean on others to populate other agency sections.

Still to do:

• Go through all files and make sure references are up-to-date (eg secret names, remove refs to clients/Access, etc).
• Test these work - will try updating one of my own client projects.
• Add more example workflows - in particular, I want to add "build a Node app and combine with a CMS build for deployment".
• Add a CONTRIBUTING.md file, as well as other metafiles (eg CODEOWNERS, PR template, etc), though these can be done at a later date.

[BenGarry-GAIN]

Assume these accessdigital references need to point to thisisgain?

[iamsophiesk]

Yes, see the "to do" list in the main PR description, this is still a draft :)