[LIN-680] Produce first revamp of contributor guide

[yoonspark]

Summary

Several users came to us asking for a way to contribute, mentioning their difficulty in doing so. This suggests our existing documentation for contributor guide is not effective. This PR includes a first round of updates to improve documentation for external contributors.

Note:

• Updates incorporate feedback from several existing contributors
• The updated RTD can be previewed here

Changes

• [x] Add a new CONTRIBUTOR GUIDE section in RTD
• [x] Add basic technical instructions to help new contributors get started (e.g., setup)
• [x] Add instructions on overall contribution process
• [x] Add recommended practice(s)
• [x] Add a tutorial on how to contribute new package annotations
• [x] Add instructions on how to annotate packages "on the fly" (i.e., without changing source code)
• [x] Reorganize USER GUIDE contents relating to artifacts
• [x] Direct the user into FAQ section for issues with incomplete/incorrect code cleanup

Testing

This is a pure docs update, so no need for automated testing. Examples discussed in docs were validated in a local setting.

Ticket(s)

• https://linea.atlassian.net/browse/LIN-658
• https://linea.atlassian.net/browse/LIN-680
• https://linea.atlassian.net/browse/LIN-697
• https://linea.atlassian.net/browse/LIN-699