Add `Mithril Threat Model` page on website

[jpraynaud]

Content

This PR includes a new Mithril Threat Model page on the website based on the original Hackmd document:

[!TIP] In order to run locally the website, please run the following steps:

• Pull this branch
• Run cd docs/website from the root of the repository
• Run npm install
• Run make dev
• Open your browser at http://localhost:3000/doc/next/mithril/threat-model
• Pages are automatically refreshed in the browser when they markdown files are modified

Pre-submit checklist

• Branch
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested
• Documentation
  • [x] Update documentation website (if relevant)
  • [x] Rotate documentation before merging

Issue(s)

Closes #1350

[github-actions[bot]]

Test Results

    4 files  ±0     51 suites  ±0   8m 45s :stopwatch: -1s 1 073 tests ±0  1 073 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  1 221 runs  ±0  1 221 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 85828437. ± Comparison against base commit 56aa23ef.

:recycle: This comment has been updated with latest results.