Edit the Resource Request guide

Closes #33417

Focus the guide more on its stated topic by moving tangentially related
reference information at the end of the guide to more appropriate pages.

- Replace the `search_as_roles` section with a link. Much of the "Next
  steps" section is a long list of examples of Teleport roles that an
  admin could assign to `search_as_roles`. This is unnecessary, since a
  user familiar with a Teleport role won't need this information to
  understand `search_as_roles`. It is not the job of the Resource Requests
  guide to teach users about Teleport roles.
- Condense the "Next steps" guidance. There are "Next steps" sections.
  Make them one. Also turn the section re: Access Request plugins into a
  bullet.
- Move instructions for searching for and requesting Kubernetes
  resources into the tsh guide. These instructions are end users, so
  move them to the appropriate guide to make them more discoverable.