feat: implement efficient backend querying on the tasks page (coder#18488)

Use the `/workspaces?q=has-ai-task=true`,
`/templates?q=has-ai-task=true` and `/aitasks/prompts` endpoints to
fetch Task templates and workspaces on the `/tasks` page.

Also:
- remove documentation link placeholders: the documentation is not in
place yet and is not going to be available before the June 24th code
freeze
- load workspaces and templates in parallel
- replace loading spinners with content skeletons

Related to coder#18454 and
coder/internal#660.

Author: hugodutka