Consolidate data FE/BE fields in stages.yml

The following discussion from !141754 (merged) should be addressed:

• @bwill started a discussion: (+1 comment)

  A lot of these fields are redundant as I think we have moved away from having different managers for FE / BE and now have a single manager for all teams (is this right?). I think we ought to consolidate these into a single field.

@cynthia While I agree, that would be a larger effort to change the data schema itself and ensure the handbook code is updated too.

There's been discussions for the purposes of listing people whether we should consolidate on engineer as well, since we have backend, frontend, fullstack, test, SRE, data, model, etc.

Minor changes to make: ensure maturity categories are updated and link to handbook page.