[Docs Migration] Update MDX syntax for Mintlify

[kapunahelewong]

To help with docs migration efforts, this PR updates syntax for Mintlify:

• Adds two blank lines after the front matter (I noticed that in [Docs Migration] Update MDX files so they are properly parsed by Mintlify. #27649 but can revert that if needed).
• Removes previous syntax such as {: .external} and {:#load}.

This PR covers a few of the files mentioned in bazel-contrib/bazel-docs#103, which links to a list of docs that need to be updated at https://github.com/bazel-contrib/bazel-docs/blob/main/copy-upstream-docs.sh.

[gemini-code-assist]

Code Review

This pull request updates several documentation files to align with the Mintlify syntax as part of a documentation migration. The changes include adding blank lines after the front matter and removing legacy syntax for external links and header IDs. The modifications are consistent and correctly applied across all files. The changes look good.

[kapunahelewong]

I've also started a draft PR to remove these files from the to-do list: bazel-contrib/bazel-docs#181