docs: move ERD diagrams to PlantUML

[steve-chavez]

Problem

We use PlantUML for almost all our diagrams, except for the ERD diagrams. This adds bloat to our Nix tooling. See:

postgrest/docs/_diagrams/README.md

Lines 1 to 11 in e20dc60

	## ERD
	The ER diagrams were created with https://github.com/BurntSushi/erd/.
	You can go download erd from https://github.com/BurntSushi/erd/releases and then do:
	```bash
	./erd_static-x86-64 -i ./er/film.er -o ../_static/film.png
	```
	The fonts used belong to the GNU FreeFont family. You can download them here: http://ftp.gnu.org/gnu/freefont/

These diagrams are on the Resource Embedding page.

Solution

Move the ERD diagrams to PlantUML. See an example here.

Notes

In the past we removed Latex in favor of PlantUML to reduce bloat (#3616).