docs: add an rstack dev how-to page #2641

Follow-up to #2635 and #2637. rstack dev is documented in products/platform/PROJECT-DEV.md, which is a product reference. The docs site has no page for it.

Add a how-to page under docs/site/src/content/docs/ that takes a developer from a checkout to a running BReg and Casework pair:

1. install the components;
2. start a local TLS PostgreSQL server (the recipe in PROJECT-DEV.md);
3. write or adapt dev.yaml from the licensing example;
4. rstack dev check, start, token with curl, stop;
5. what to do when a run fails (logs, recovery).

Link it from the CLI reference and from PROJECT-DEV.md, and verify it as a reader on a clean machine.