Add deprecated API rate limit page to left nav

When gitlab!70310 (merged) merges (likely while Amy is on PTO), this work needs to be reviewed and (hopefully) merged. I'm adding @nick.thomas as a reviewer because gitlab!70310 (merged) is his work.

I followed the prior art in !2157 (merged) and added this page to the same subsection of the nav that I added another API rate limit page into.

When it merges, ping the link to this MR in the #docs channel for help from another technical writer.

• Link to the merge request of the new page: gitlab!70310 (merged)

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page. gitlab!70310 (merged)