Add promotion note for GL.com users

What does this MR do?

As per title/commit message, MR adds a note that feature is temporarily free for GitLab.com users.

Related issues

Closes #9545 (closed)

Author's checklist

• Apply the correct labels and milestone

N/A:

• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required) @eread
• Merge the EE-MR first, CE-MR afterwards