Add PVF module documentation (#6293)

* Add PVF module documentation

TODO (once the PRs land):

- [ ] Document executor parametrization.

- [ ] Document CPU time measurement of timeouts.

* Update node/core/pvf/src/lib.rs

Co-authored-by: Andrei Sandu <[email protected]>

* Clarify meaning of PVF acronym

* Move PVF doc to implementer's guide

* Clean up implementer's guide a bit

* Add page for PVF types

* pvf: Better separation between crate docs and implementer's guide

* ci: Add "prevalidating" to the dictionary

* ig: Remove types/chain.md

The types contained therein did not exist and the file was not referenced
anywhere.

Co-authored-by: Andrei Sandu <[email protected]>