Deprecate public `coder/modules` repo

[f0ssel]

This issue hinges on #531 being completed.

Once all modules have been migrated to the central Hub repo, we'll need to officially deprecate the coder/modules repo. Not fully sure about everything that would go into that, but this also seems like an issue that would be more for Product.

[buenos-nachos]

Most of work needed to flag the modules repo as deprecated is done now, but the last piece is making sure we migrate all the Bash scripts and CI processes (some are already, but not all). Probably going to spend Tuesday working on that, and then use Wednesday/Thursday to start on the Registry site updates

[buenos-nachos]

Making a quick checklist of work that still needs to be done before we can consider deprecating the modules repo:

• Add new (marketing-approved) top-level README
• Get site health Cron script working
• Copy over deploy-registry.yaml (and possibly tweak it so that the user flow is more obvious)
• Copy over logic for checking for typos and un-prettified code to ci.yaml
• Copy over/recreate CONTRIBUTING.md
• Recreate new.sh script to make it easier to get started with the Registry
• Copy over/recreate release.sh script
• Copy over/recreate terraform_validate.sh script

All items with linked PRs should hopefully be done once those PRs are merged in

[buenos-nachos]

@matifali Do you have the bandwidth to be able to help out with updating the new.sh and release.sh Bash scripts?