docs: refresh outdated model examples and add Chat Server page

[dgageot]

What

This PR fixes a few pieces of the docs that were outdated, plain wrong, or missing.

Outdated / wrong

• Recommended Anthropic model – the docs were inconsistently using both claude-sonnet-4-0 (older) and claude-sonnet-4-5 (the current recommended default per docs/providers/anthropic/index.md). All example configs are now normalized to claude-sonnet-4-5. The Anthropic provider page still lists claude-sonnet-4-0 in the Available Models table as the previous generation.
• Broken external link in docs/providers/openai/index.md – modelname.ai returns a 404 ("This app isn't live yet"). Replaced with the correct modelnames.ai and added a link to the official OpenAI models doc.
• Bedrock IAM-role example used the long-deprecated anthropic.claude-3-sonnet-20240229-v1:0 model id while every other example on the same page uses the current global.anthropic.claude-sonnet-4-5-20250929-v1:0. Made the IAM section consistent.

Missing

• Chat Server feature page – docker agent serve chat (the OpenAI-compatible chat-completions server) was documented as a CLI command in the CLI reference but had no dedicated feature page and no entry in the side navigation. Added docs/features/chat/index.md covering:
  • the /v1/models, /v1/chat/completions, and /openapi.json endpoints
  • quick-start with curl and the OpenAI Python SDK
  • server-side conversation caching (X-Conversation-Id)
  • bearer-token auth and CORS
  • Open WebUI integration recipe
  • the full flag table for docker agent serve chat
• Wired the new page into docs/_data/nav.yml and added cross-links from the API server page and CLI reference.

How verified

Docs-only changes. Verified:

• docs/_data/nav.yml is still valid YAML.
• All updated model names are still recognized by the codebase (claude-sonnet-4-5 is the recommended default in docs/providers/anthropic/index.md, global.anthropic.claude-sonnet-4-5-20250929-v1:0 matches the rest of the Bedrock page).
• modelnames.ai resolves; modelname.ai does not.

Files

• New: docs/features/chat/index.md
• Modified: docs/_data/nav.yml, docs/index.md, plus 14 other doc files updating model names and adding cross-links.

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

Docs-only PR. Model name normalizations, the broken-link fix, and nav/cross-link updates all look correct. One factual inaccuracy was found in the new Chat Server page.

[docker-agent]

[MEDIUM] Introductory claim contradicts the endpoint table

Line 47 states:

"All endpoints live under the /v1 prefix to match the OpenAI API surface."

But the table immediately below lists /openapi.json at the root level — not under /v1. This is confirmed by the actual server implementation in pkg/chatserver/server.go:

// Register /openapi.json *before* the bearer-auth middleware so the
// spec is publicly accessible.
e.GET("/openapi.json", s.handleOpenAPI)

The introductory sentence is factually incorrect. Consider changing it to something like:

"The two core API endpoints follow the OpenAI API surface under the /v1 prefix. The OpenAPI spec is served separately at /openapi.json."

Or simply remove the "all endpoints" claim and let the table speak for itself.