Inconsistent preference for singular/plural in the docs

### Summary

The documentation for v5 gives recommentations how users shall design their schema and name database objects. The docs are inconsistent and sometimes recommend singular (e.g. "vehicle") and sometimes plural (e.g. "vehicles").

### Additional context

- Plural Example: https://github.com/graphile/crystal/blob/main/postgraphile/website/postgraphile/required-knowledge.md?plain=1#L57-L58
- Singular Example: https://github.com/graphile/crystal/blob/main/postgraphile/website/postgraphile/postgresql-schema-design.mdx?plain=1#L144 (more complex case "people/person")

It might be worth scanning the whole docs to make sure this is consistent.

If I remember correctly, the old advice (from v4 or the Starter) was to use singular, at least that is what I did in a couple of projects ever since 😅

PS: Looking forward to starting a brand new project with v5 in the next weeks (also with worker and migrate) – really happy with all your work, keep it up!