[Docs] Deprecation policy, SDK tier target, and CONTRIBUTING.md (#472)

* [Docs] Add deprecation policy doc (SEP-2596)

Formalizes the Active -> Deprecated -> Removed lifecycle and the
twelve-month deprecation window the SDK already applies via SEP-2577,
and states the SDK's own BC promise ahead of 1.0.

* [Docs] Add SDK Tier 2 gap analysis (SEP-1730)

* [Docs] Wire deprecation policy and SDK tier docs into the guide nav

Adds both pages under a new Project section in mkdocs.yml, links them
from index.md's "Where to go next", and points their repo-root
references at GitHub URLs instead of paths escaping docs/ (dead under
Zensical's --strict build).

* [Docs] Add CONTRIBUTING.md

Root-level contribution guide: dev setup, the make ci checks to run
before a PR, coding standards, testing expectations, the docs-guide
link-resolution rule Zensical enforces, and licensing terms. Linked
from README's existing Contributing section.

* [Docs] Trim deprecation policy per review

Drop the worked SEP-2577 example and the "relationship to the
protocol lifecycle" section; both were redundant restating of detail
that lives in the code/CHANGELOG and protocol-versions.md already.

* [Docs] Cite the Tier 3 audit and add a versioning policy (SEP-1730)

Replace our own hand-rolled conformance numbers with the authoritative
audit in modelcontextprotocol/modelcontextprotocol#3274: SDK is
currently Tier 3, blocked from Tier 2 by client OAuth conformance
(20%) and no stable 1.0.0+ release. Add the Versioning section the
audit found missing — SemVer commitment pre-1.0, Symfony's BC promise
after — and point CONTRIBUTING.md at it.

* [Docs] Link Symfony's BC promise from the deprecation policy

Was plain text; sdk-tier.md's Versioning section already links it.

* [Docs] Drop the "process gaps addressed" aside from sdk-tier.md

* Apply suggestion from @chr-hertel

* Apply suggestion from @chr-hertel

mkdocs.yml

@@ -54,6 +54,9 @@ nav:
       - Events: advanced/events.md
       - Protocol extensions: advanced/extensions.md
       - Custom message handlers: advanced/custom-handlers.md
+  - Project:
+      - Deprecation policy: deprecation-policy.md
+      - SDK tier target: sdk-tier.md
   - Examples: examples.md
   - API Reference: https://php.sdk.modelcontextprotocol.io/api/