docs: restructure and update documentation for 2.5.0

Split the README into a short front page and task-oriented pages under
docs/: installation, user guide, CLI reference, configuration and
troubleshooting, with an index in docs/README.md. Rewrite RECOVERY.md
around Continue This Backup Here and Restore Folders, and DEVELOPMENT.md
around running, debugging and releasing from source.

Bring ARCHITECTURE.md up to date with the read path, push events, the
thread map, conflict copies and the machine profile, and move the feature
design notes there. Collect planned work in ROADMAP.md. Link the threat
model from SECURITY.md and the translation rules from CONTRIBUTING.md.

Add scripts/publish-wiki.sh, which mirrors the user pages to the GitHub
wiki with rewritten links and only pushes with --push.

SECURITY.md

@@ -30,5 +30,10 @@ the interesting boundaries are:
 - **Cryptography.** Key derivation, signature handling, and anything that could publish plaintext
   or the wrong key material to Proton.
 
+What the client deliberately keeps in plaintext on disk, and why, is documented in the
+[threat model](docs/ARCHITECTURE.md#8-threat-model-what-this-client-writes-to-disk-in-plaintext).
+Behaviour described there is known and not a vulnerability by itself; a way to make it worse
+than described is.
+
 Out of scope: vulnerabilities in Proton's own service (report those to Proton AG), and issues that
 require an attacker who already has your user account on the machine.