Improve submodule command info in the docs site

[jbeveland27]

Some things to possibly add to documentation based on experiences with PRs so far.

• Add example commands for adding and updating submodule to the add-checklist and update-checklist pages. They're present in the process outline on the home page, but might be helpful to have them quickly reference-able if contributors skip directly to the Add Nerdpack/Update Nerdpack page.
• Add submodule section to Reference Docs
  • Possibly detail how to update submodule in the same PR to avoid contributors feeling like they need to close and open a new PR when changes are requested
• Add detail that submodules URLs should be HTTPS rather than SSH on the Add Nerdpack checklist and Reference Docs
• (Possibly) Add detail about automation to Reference Docs
  • Detail checks for the PR, including how it also runs when a PR is updated with new commits
  • Deploy process