Guide for first-time contributor

[ekzhu]

Refactor the CONTRIBUTING.md file to include the following:

1. Steps to get familiar with the framework
2. Suggestions of apps or extensions to build, and instructions to tag the repo with autogen-sample or autogen-extension for discovery. Share in Discord and GitHub discussion etc.
3. Sections on framework development, contributions.

Link to the CONTRIBUTING.md from README.md and Homepage prominently. Also link from GitHub Discussion and Discord as part of the welcome message