Draft: Gpg key api docs

What does this MR do and why?

This MR updates the curl sample for docs for Adding GPG key API.

• https://docs.gitlab.com/ee/api/users.html#add-a-gpg-key
• https://docs.gitlab.com/ee/api/users.html#add-a-gpg-key-for-a-given-user

Because currently there is no mention of URL encoding of the request body, so copying and pasting this sample will result in an error.

Screenshots, screen recordings, or links to review app

Example for not URL encoded

Example for URL encoded 1

Example for URL encoded 2

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.