Docs for security model AND architecture diagram

[mreeve-snl]

🗣 Description

Merges in Chages to docs to add Security Model Adds in the 2.0 architecture diagram to docs. Relevant links in the Main readme have been added

💭 Motivation and context

See above

🧪 Testing

N/A

✅ Pre-approval checklist

• [x] There is a gitIssue that this PR resolves
• [x] Git Issue that this PR solves has been selected in the Development section
• [x] The PR's base branch has been modified to be the proper branch.
• [x] Changes are limited to a single goal AND the title reflects this in a clear human readable format
• [x] Issue that this PR solves has been selected in the Development section
• [x] I have read and agree to LME's CONTRIBUTING.md document.
• [x] The PR adheres to LME's requirements in RELEASES.md
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.

✅ Pre-merge Checklist

• [ ] All tests pass.
• [ ] PR has been tested and the documentation for testing is above.
• [ ] Squash and merge all commits into one PR level commit.

✅ Post-merge Checklist

• [ ] Delete the branch to keep down number of branches.
• [ ] The PR is labeled with feat for an added new feature, update for an update, OR fix for a fix.
• [ ] The PR contains Resolves #<issue #> so that merging it closes out the corresponding issue. For example Resolves #132.

[mreeve-snl]

the diagram won't load because it hasn't been merged, but the URL is setup so it should be available on merge to release-2.0.0: https://github.com/cisagov/LME/blob/mreeve-406-threat-model/docs/imgs/lme-architecture-v2.jpg

[mreeve-snl]

this is covered by clints new PR: #458 cbaxley-352-vault-user-password-encryption closing this PR