Update agent token docs to point at tokens page

[sj26]

We're working on a new agents page in the dashboard which will include a new agent tokens page:

So the content of this page can be simplified.

The new page is not yet released, and this branch should not be merged until general availability.

Fixes EDGE-411.

[render[bot]]

Your Render PR Server URL is https://bk-docs-pr-1790.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-ce6p73hgp3jgu66hm910.

[sj26]

This was included in the march release, so is ready to ship.

[eleanorakh]

@sj26 would you like me to take over getting this one over the line?

[sj26]

@eleanorakh yes please! 🙏