Add current_ref placeholder for markdown documentation

What does this MR do and why?

This MR adds a new %{current_ref} Markdown placeholder.

When the markdown_placeholders feature flag is enabled, %{current_ref} is replaced with the ref that the Markdown is being rendered for:

• Branch name when viewing a branch
• Tag name when viewing a tag
• Commit SHA when viewing a specific commit

This makes it easier to write documentation (for example, coverage badges) that always point to “the ref I’m currently looking at” without hard-coding a particular branch name.

References

• #27093
• #14389

How to set up and validate locally

1. In a Rails console, enable the feature flag:

   Feature.enable(:markdown_placeholders)

2. In any project, create an Markdown file and add a line such as:

   Current ref: %{current_ref}

3. View that Markdown on different refs:

   • On the default branch.
   • On another branch.
   • On a tag.
   • On a specific commit.

4. Confirm that:

   • On a branch view, %{current_ref} is replaced with that branch name.
   • On a tag view, %{current_ref} is replaced with the tag name.
   • On a commit view, %{current_ref} is replaced with the commit SHA.

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