Add how to install watch fixtures yourself to the docs

[jcgsville]

When setting grafserv: { watch: true } and omitting makePgService's superUserConnectionString, Postgraphile errors with the following text:

Installing watch fixtures requires superuser privileges; have you correctly configured a 'superuserConnectionString'?
You may also opt to configure 'installWatchFixtures: false' and install them yourself.

(Great error message, btw! Clear, helpful errors are an underrated part of UX 🙂)

When using Postgraphile myself, I want to install watch fixtures as a part of my db setup so that the server running postgraphile does not need access to super user db creds. I went to check the docs on how to do this, and noticed it wasn't in the docs anywhere. We should add how and why to do this to the docs.

I found the queries here: https://github.com/graphile/crystal/blob/36c41e80bfe312f584af42545688fb78d3adaf3f/graphile-build/graphile-build-pg/src/watchFixtures.ts

A quick 👍 to make sure we're aligned, and I'm happy to make the PR for this addition myself.

[jcgsville]

Relatedly, as I'm adding these watch fixtures to my migrations, I'm realizing that I need to keep an eye on them any time I update PostGraphile to be sure they aren't changing. While I highly doubt these will change much, if ever, I think it may make sense to have a way to do "watch fixture migrations only" via postgraphile, such that the schema is updated automatically. I think Graphile Worker has similar functionality, though worker's schema changes far more regularly, of course, so it's more useful

[benjie]

Yes please, though we should tell people how to output the SQL to a file and tell them to check it hasn’t changed on each update since it’s not the supported approach.