docs: virtualize crate READMEs under docs/components/

Each workspace crate's own README.md now also shows up as
docs/components/<crate>.md via nix/packages/reference-docs.nix, so it
renders on the docs site alongside the rest of docs/ without a second
hand-copied file to keep in sync. Relative links that were correct
from the crate's own position in the tree are rewritten for their new
virtual position (../docs/x.md -> ../x.md, ../<sibling-crate> ->
./<sibling-crate>.md). Landing page at docs/components/README.md, nav
bullet added to docs/README.md. No changes needed in the website
repo's docs.nix - it already walks every subdirectory generically.

docs/README.md

@@ -86,6 +86,13 @@ declarations.
 - **How do I export Claude Code metrics (tokens, cost, tool calls) to
   Prometheus/Grafana?** → [`observability.md`](observability.md).
 
+## Crate reference
+
+- **What does a specific Rust crate do, on its own terms?** →
+  [`components/`](components/README.md) — every workspace crate's own
+  `README.md`, one level up from source (hyperhive#3051); the crate
+  itself is still the source of truth, this is just a walkable mirror.
+
 ## Process & conventions
 
 - **Naming, commit style, wire protocol, the `data-async` pattern?** →