Multiple website updates

[mehaase]

This PR contains 3 tickets:

1. MAPEX-170: adds the VERIS - ATT&CK Group mappings. This is quick and dirty to get them onto the website, and I opened up another ticket to improve handling of ATT&CK Groups in the future. If you go to the VERIS landing page (for 1.3.7 and 12.1), you'll see a link to download the ATT&CK Group mappings in the original JSON format (note: not in the mapex schema.)
2. MAPEX-141: update the README to describe various modes of public contribution.
3. MAPEX-163: add project scope/methodology documents to framework landing pages.

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[mehaase]

@tiffb Can you take a look at this? A few things to look at:

• I added pages to the "About" section for:
  • CVE Methodology
  • NIST Methodology
  • NIST Scope
  • SSM Methodology
  • VERIS Methodology
• The new pages are also linked from their respective framework pages, e.g. the CVE page links to the CVE methodology.
• The VERIS groups file can be downloaded from the VERIS page.
• Updated the README to include some more contribution details (and also re-arranged the existing content to match our other projects)