Overview Chapter Followup

This issue exists to track things we want to improve on. Please add things here:

• Replace bullets with prose.
• Perhaps move some of the content about command line parsing, lexing, and parsing to the appropriate chapters.
• More info on type checking and type inference.
• Fix some of the TODOs
• add more links to other parts of the guide
• add diagrams
• answer unresolved questions at the bottom
• figure out where the following happen and add them back to the list of steps
  • type checking
  • trait solving
  • const eval
  • size optimizations
• Perhaps use more content from https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/history.20of.20typeck.20and.20borrowck about HAIR
• find out if there are other reasons to use query system beyond incremental
• improve descriptions of bootstrapping.
• verify info about parallel rustc effort

Start with the rustc-dev-guide Overview chapter and review each open checklist item, including the sections on command-line parsing, lexing, parsing, type checking, trait solving, const evaluation, and size optimizations. Done means resolving specific TODOs and unanswered questions, adding links or diagrams where needed, verifying the parallel-rustc information, and updating the checklist.