docs: document server.yml support and plumber2 content items #386

Upstream: ricochet-rs/ricochet#1416, feat(core): support server.yml standard and thus plumber2

ricochet now reads the server.yml standard, which is what makes plumber2 APIs deployable.

Neither server.yml nor plumber2 appears anywhere in src/content/docs/dev/.
The R content type table in user/content-items/1-content-items-overview.mdx lists plumber but not plumber2, and user/deployment/4-r.mdx never mentions the file.

Pages: user/content-items/1-content-items-overview.mdx, user/deployment/4-r.mdx