docs: Fix transform-docs.awk to use dashes in anchor references

[promptless]

Fixes the transform-docs.awk script to convert underscores to dashes in all anchor references, ensuring compatibility with Mintlify's auto-generated anchors. This resolves broken links in reference documentation like /reference/be/c-cpp where rule names (cc_binary, cc_library, etc.) were not linking correctly to their sections.

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[alan707]

@Promptless, if you attempt to click on the link for cc_binary in the main page of https://bazel-promptless-fix-anchor-underscores-to-dashes.mintlify.app/reference/be/c-cpp you will notice that it still has an underscore instead of the correct dash. The reason is that we need to modify the the YAML file to actually generate those MDX files in this PR and push the commit. Similar to how the automerge.yaml workflow works

[promptless]

I've added the requested changes in commit 55228d0