Create .mdx docs for Bazel 9.1

### Description of the feature request:

Same problem as https://github.com/bazelbuild/bazel/issues/28930 : we need to create .mdx docs for the upcoming 9.1 release.

IIRC most of the docs changes are not part of the 9.x branch, so we probably need to run the old .md docs through the docs2mdx pipeline at HEAD, then use Gemini CLI to fix the various syntax errors and bring back custom header IDs.

### Which category does this issue belong to?

_No response_

### What underlying problem are you trying to solve with this feature?

_No response_

### Which operating system are you running Bazel on?

_No response_

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### Have you found anything relevant by searching the web?

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### Any other information, logs, or outputs that you want to share?

_No response_