docs: POSIM migration, candidate Docker Quickstart and backend limits

[yeseorizi]

Summary

Documentation-only follow-up: distinguish the published PR #5 validation candidates from unmerged source work and future releases.

• Add a documentation index, first-user Quickstart, backend support matrix and DAVE-to-POSIM notice.
• Document both public validation tags and immutable registry digests, exact installed workspace setup, headless world launch and clock/shutdown checks.
• Keep native Ubuntu source installation separate from the tested Docker candidate. Retain dave_* package/launch/workspace compatibility names.
• Point WGPU and WAM-V review to POSIM Migrate WGPU sonar backend from DAVE #44 #6 and Migrate ocean waves and WAM-V from DAVE #27 #7, without claiming they are included in main or the published images.
• Clarify that general publication remains disabled and GUI/RDP, joystick, CUDA/WGPU sonar and all-world coverage are outside the headless acceptance scope.

Validation

• git diff --check: PASS.
• 22 Bash blocks: bash -n PASS.
• 35 relative links: existing destinations verified.
• 10 documented launch commands: arguments checked against main's launch declarations.
• Candidate Quickstart commands: matched to saved PASS commands for both architectures from 2026-09-30 registry re-pull artifacts.
• External URLs checked; no HTTP errors in the audit.
• This is not a new clean-host native installation or a rerun of the full image matrix.

Wiki

The working personal Wiki now has a new POSIM guide and redirect/status notices on the installation, Docker, native, system requirements, sonar and migration pages. Earlier DAVE material was retained as historical reference.

https://caring-dibble-be5.notion.site/3ecc941998988150ad59f75d5bd105cf

The original dave-ros2.notion.site home was also updated through its authorized Notion editor. Its public view now displays the POSIM transition notice, new repository/guide links, candidate-image status and WGPU development limitation. Historical child pages were retained; this is not a claim that every legacy tutorial has been rewritten or re-executed.

Safety / review

• Do not merge automatically. PR Validate POSIM images and fix Docker runtime shutdown failures #5 remains open and is not changed by this PR.
• Based on current main e05a9fb7, not on the candidate branch: no runtime, Dockerfile, workflow, secret, variable, image or release-tag changes.
• [skip ci] prevents expensive native image builds triggered by a README-only documentation change. Static documentation checks above were run locally; no CI success is claimed for this docs commit.
• All facts are dated 2026-10-01; update branch/release status when those PRs are actually merged.