impl guide: update PVF host page; add diagrams

[mrcnski]

Would have been nice to have some helpful diagrams a while ago, but I just thought of it. :P
cc @eagr @jpserrat

Screenshots

[eagr]

would be nice if you could also post the clips of the diagrams here :))

[s0me0ne-unkn0wn]

   ┌─ node/utility/pvf-prechecker.md:11:110
   │  
11 │   This subsytem does not produce any output messages either. The subsystem will, however, send messages to the [Runtime
   │ ╭──────────────────────────────────────────────────────────────────────────────────────────────────────────────^
12 │ │ API] subsystem to query for the pending PVFs and to submit votes. In addition to that, it will also communicate with
   │ ╰────^ Did you forget to define a URL for `Runtime API`?
   │  
   = hint: declare the link's URL. For example: `[Runtime API]: http://example.com/`

Would be good to fix it as well. Otherwise, looks very good!

[s0me0ne-unkn0wn]

Probably a random place to ask, but did anybody manage to get a usable PDF of the Implementer's Guide out of mdbook? I tried the printing function and mdbook-pdf crate, but the diagrams are broken af, they're not auto-scaled, and a lot of them are cut in half.

[mrcnski]

@eagr See the "Screenshots section. 😀

@s0me0ne-unkn0wn No idea, but even in HTML mode the diagrams are a bit broken, I need to scroll a lot to see the bigger ones... And it's nice that they are generated from code inline with the docs, but they're a bit ugly haha.

[eagr]

Suggested change

	pp [label = "Prepare\nPool"; shape = square]
	pp [label = "Prepare\nPool"; shape = square]
	pq [label = "Prepare\nQueue"; shape = square]
	pvf -> pq [label = "Prepare"; style = dashed]
	pq -> pp [label = "Prepare"; style = dashed]

Missing the prepare queue?

[mrcnski]

Yeah, will fix. In my head the queue and pool do the same thing, and I'm not sure why they are separate. I think I asked at one point, and the pool was supposed to fulfill some role that is actually no longer needed, but I don't remember.

Making this diagram also had me asking why Candidate Validation sends messages to the PVF host, when we could have instead embedded the host in the subsystem. It seems a bit over-engineered, and the extra message passing is inefficient, but it's cold code so 🤷