docs: add "Wiring pyrer into rez" integration guide

[doubleailes]

Summary

`pyrer` is the solver hotpath only — `rez` still owns discovery, env build, the `ResolvedContext` lifecycle. The README has the basic `pyrer.solve` API but doesn't say how to actually slot it behind a normal `rez env` flow. This PR fills that gap.

What's in the new docs page

`docs/content/docs/getting-started/rez-integration.md`:

• The integration model: what pyrer is and isn't, with an ASCII pipeline diagram showing where rez stays and where pyrer takes over.
• `build_pyrer_repo(package_paths)`: walks `rez.packages.iter_package_families` and produces the JSON shape `pyrer.solve` expects. ~15 lines.
• Solving: calling `pyrer.solve` with the result shape and the three `status` values.
• Result translation back to rez: how to turn `(name, version, variant_index)` tuples into rez `Variant` objects via `rez.packages.get_package(name, version).get_variant(idx)`.
• A complete monkey-patch shim: minimum-viable replacement of `rez.resolver.Resolver._solve` that delegates to pyrer on the happy path and falls back to rez's own solver for any non-default config (custom orderer, filters, late-binding requires). Drop-in via `sitecustomize.py` or `plugin_path`.
• Caveats: explicit list of what pyrer does not model yet — `VariantSelectMode::intersection_priority` (Solver: implement VariantSelectMode::intersection_priority #63), `@early` / `@late` requires, custom `PackageOrder` / `PackageFilter`, variant-index parity in the differential.
• A sanity-check loop for diffing pyrer vs rez on the user's own repo.
• Note on running many resolves against the same repo: build the dict once, plus pointer to pyrer's shared variant cache (perf(resolver): allow sharing the variant cache across solves #66).

Other changes

• Link the new page from the introduction's "Next steps" list.
• Compressed reference block in the README's "Using it from Python" section — the same three-step walk → solve → translate, with a link to the full docs page.

Verification

• `zola build` — 16 pages, 8 sections, no warnings.
• `zola check` — no broken links.

🤖 Generated with Claude Code

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