Document CI/CD configurations and build scripts

[pelesh]

Create four README files for:

• GitHub runners
• GitLab runners
• Build scripts
• Spack package configurations

Probably a good idea to create markdown or ReST files with documentation in respective directories.

[cameronrutherford]

Also need to write small workflow guides when you might want to:

• Rebuild resolve deps
• Update resolve spack package / add a new version
• Refresh secrets / modify GitHub workflows files to change behavior

[cameronrutherford]

I am pausing work on this until #44 is merged. cc @ryandanehy

[pelesh]

I am pausing work on this until #44 is merged. cc @ryandanehy

@cameronrutherford, it is merged. We can continue 😉

[pelesh]

@cameronrutherford, @ryandanehy: Please see also #80, which is related to this issue, and suggest how to handle it.