Fill Under Construction stubs by moving content from server.md:
- authorization.md: server-side TokenVerifier + client-side OAuthClientProvider
  (moved from server.md and client.md)
- low-level-server.md: low-level Server API, lifespan, structured output,
  direct CallToolResult, pagination (moved from server.md)

Delete concepts.md (redundant with server.md core concepts section).

Update mkdocs.yml nav to include server.md, client.md, protocol.md
(previously missing from deployed docs site).

server.md: 2189 -> 1635 lines.

- Fix Icon sizes type: str -> list[str]
- Remove fabricated validation_error field from ElicitationResult docs
- Fix isinstance check: TextContent -> TextResourceContents for resource contents
- Update all spec version links to 2025-11-25