Update to bevy 0.19

[Jengamon]

• this is more a subtractive change b/c in 0.19, render graphs no longer exist, it's Schedules all the way down. (This also means that we don't need bevy_render as an optional dependency anymore.)
• we point at main branch b/c there was a change made in 0.19-rc.3 that makes ambiguity edges impossible, but is fixed by give immutable access to SystemWithAccess::system bevyengine/bevy#24652

Large diff b/c regenerated README docs (can split out if desired)
For actual code changes, filter out .svg and .dot files

[Jengamon]

For kicks, a nushell one-liner to generate all missing svgs within a schedule subfolder:

ls | where name =~ ".dot$" | upsert svg_name {|r| [$r.name, "svg"] | str join "." } | where {|r| not ($r.svg_name | path exists) } | each {|r| ^dot -Tsvg $r.name o> $r.svg_name}

[jakobhellermann]

Thanks!