docs: document reserved process types

[dmonroy]

`process_type` is used throughout the CLI reference (`ps resize`, `ps scale`, `ps restart`, `logs`) but was never defined anywhere in the docs, and a search for the term itself returned no matching documents.

Two changes to `apppack.toml`:

• `[services.]` now says explicitly that `` is what the CLI calls a process type.
• New "Reserved process types" section documenting `release`, `scheduler`, `shell`, `postdeploy`, and `pr-predestroy`, the process types AppPack creates itself rather than ones defined under `[services.]`, plus a note that `[test]` isn't a process type at all since it runs inside the build container, not as a separate ECS task.