New Page for Joining SLIM

[riverma]

Purpose

• New stand-alone page documenting how to get-involved or join SLIM, making it much easier (and more likely) for project teams to join.
• FEEDBACK WELCOME: https://github.com/NASA-AMMOS/slim/tree/get-involved/docs/join

Proposed Changes

• [ADD] New Join page
• [CHANGE] Migrated Get Involved content from About page to new join page

Issues

• N/A

Testing

• Tested on local instance of Docusaurus

[riverma]

Sorry for the short time to review folks! Need this for a demo tomorrow ;). Please post a new PR or issue ticket if you have concerns with this PR.