docs: add LLMTR provider

[knowhycodata]

Issue for this PR

Closes #42866

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds the missing LLMTR section to the provider directory in providers.mdx. One file, docs only.

LLMTR already works in opencode — it is in models.dev and live in the catalog mirror (llmtr, npm @ai-sdk/openai-compatible, env LLMTR_API_KEY), so it resolves through the generic openai-compatible path with no runtime code. The only thing missing was the docs entry, so this is purely that.

I put it after LLM Gateway and before Poolside, and followed the same four-step connect/models shape the neighbouring entries use. The config snippet uses the existing "add a model to an existing provider" pattern from the OpenRouter and LLM Gateway sections rather than any new syntax, because the models.dev snapshot only carries the Turkey-hosted llmtr/* ids — anything else from the gateway has to be named in config.

How did you verify your code works?

Docs build, on this branch:

• bun run --cwd packages/web build → [build] Complete!, exit 0. The section renders at /docs/providers with anchor id="llmtr".
• prettier --check packages/web/src/content/docs/providers.mdx → All matched files use Prettier code style!
• bun run lint (oxlint) reports one error and 4855 warnings, but they are all pre-existing on dev and in .ts/.tsx files; oxlint does not process .mdx.

Provider behaviour, on the released opencode-ai@1.18.18 with LLMTR_API_KEY set:

• opencode models lists the llmtr/* catalog models.
• opencode run --model llmtr/gemma-4 "Reply with exactly: OK" → OK.
• With the exact config snippet from this PR in opencode.json, opencode models gains llmtr/anthropic/claude-opus-4.8 and llmtr/openai/gpt-5.4, and opencode run --model llmtr/anthropic/claude-opus-4.8 returns a real response. Both ids were also checked directly against POST /v1/chat/completions (HTTP 200).

I deliberately did not name llmtr/sincap anywhere: it is retired and returns HTTP 410. It is still in the models.dev snapshot; a separate PR fixes that catalog drift.

Screenshots / recordings

Not a UI change — content added to an existing docs page.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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Context: this was previously #31315 / #31316. The PR was closed by the automated PR-cleanup bot and the issue by the 60-day stale bot, not on technical merit, and neither can be reopened, so this is a refile against current dev.