feat: add Relevanssi for improved searching

[greatislander]

• [x] I've read the guidelines for Contributing to the Platform Co-op Toolkit
• [x] This isn't a duplicate of an existing pull request

Description

Adds Relevanssi. This adds support for searching by topics/goals etc.

Steps to test

Not testable in and of itself; configuration needed.

Additional information

Not applicable.

Related issues

Not applicable.