Cut the 0.2.0 release

[oubiwann]

Tasks:

• [x] Checkout local release/0.2.x branch and rebase from remote for latest
• [x] Check README / docs for any remaining changes necessary
• [x] Update versions:
  • apps/undertone/src/undertone.app.src
  • src/undertone-lib.app.src
  • relx entry in rebar.config
• [x] PR release/0.2.x or merge/push to main
• [x] Checkout local main and rebase from remote main
• [x] Tag and push
• [x] Close out milestone
• [x] Create a new release branch for the next dev cycle
• [x] Bump all three version sources for the next dev cycle and push
• [x] Set the default branch to the next release cycle at https://github.com/lfex/undertone/settings/branches