Add Search Feature to Mentors Page

- [ ] User must be able to search by name, city, or topic.
- [ ] The search results page must show the number of found mentors.
- [ ] The search results page must be paginated.
- [ ] The search results page must have a permalink.

Start by locating the Mentors page and the code that currently lists mentors, then trace how its requests and results are rendered. Confirm how search, pagination, and permalinks are handled elsewhere in the project before making changes. Done means users can search by name, city, or topic, see the result count, navigate pages, and share a permalink.

Written by the indexing model from the issue text.