Follow-up to #4194 (v3 execution app + core-helper removal). The v3
(engine V1) is end-of-lifed and enforced off in prod, so this removes a
self-contained slice of the remaining dead v3 code while **keeping every
user-facing deprecation message** - a user still on v3 must still be
told to upgrade.
## Legacy dev websocket
`app/v3/handleWebsockets.server.ts` backs the `/ws` transport used
**only** by the legacy v3 `trigger dev` CLI (v4 dev uses a different
transport). It's now authenticate-then-close with
`V3_DEV_DEPRECATION_MESSAGE`, so an old CLI is still told what to do -
only the legacy `AuthenticatedSocketConnection` / `DevQueueConsumer`
execution behind it (which can no longer run) is removed.
- Deleted `app/v3/authenticatedSocketConnection.server.ts` (its only
consumer).
- `engineDeprecation.server.ts` and the deprecation message constants
are untouched.
## Docs
Deleted the intentionally-legacy "Docker (legacy)" self-hosting page
(`open-source-self-hosting.mdx`) and redirected
`/open-source-self-hosting` (+ the existing
`/v3/open-source-self-hosting` alias) to `/self-hosting/overview`;
repointed the two inbound links. The current `self-hosting/*` docs
already describe the v4 (single supervisor) setup.
## Deliberately out of scope
Despite the branch name, this PR does **not** touch MarQS or the
socket.io coordinator/provider namespaces. Investigation found MarQS is
entangled with **live v2** queue/metrics/concurrency/project-cleanup
code (`runQueue`, `queueSizeLimits`, `taskRunConcurrencyTracker`,
`EnvironmentQueuePresenter`, `registerProjectMetrics`, `deleteProject`),
so it needs a per-file reviewed pass, not a bulk delete. That remainder
stays on TRI-11883.
refs TRI-11883
* Make the tooltip text color grey so it’s readable again the primary color
* Setup and styling for the Guides section
* Better Guides icon
* Improved the prerequisites
* WIP Nextjs guide and new folder structures
* Copy updates
* Useful next steps is now a snippet
* Better icons for the next steps section
* Renamed the “prerequisites” snippet
* Node.js guide
* Added a “Creating a project” guide
* New snippet for prerequisites
* New Remix guide
* Added Remix to the side bar
* Tweaked icons for creating a project page
* Added the hello world step to the onboarding steps
* Better “useful next steps” snippet card links
* Moved prerequisites
* Fixing links to images
* Moved v2 migration page to guides
* Removed dead link
* Attempt fix for redirect
* Fixed redirects
* Getting started section includes link to roadmap
* Removed icon from side menu
* Deleted all v2 pages (excluding updating mint.json)
* Removed v2 pages, redirects and versions from mint.json
* Deleted v2 snippets
* Deleted un-used pages
* Set of more useful coming soon snippets
* All snippets use the updated format
* updated folder “v3/“ with “/pages”
* Moved all main docs files to the route and updated the redirect
* Fixed URLs in the mdx pages to the new route path
* URL goes to the proper pricing page
* Better delayed runs image
* Attempt fix for self hosting page not redirecting
nicktrn
