feat: relax file requirements, improve IO errors

[shadows-withal]

Behaviour changes:

• Readme file doesn't need to exist anymore, is now optional, with a warning being output
• oranda now hard errors when a hardcoded mdbook path can't be found
• local logo paths will now error when they're not found
• additional pages paths will also error when they're not found

TODO:

• Add backticks around the path in the error message for readability
• Update docs to mention that you don't need a readme circa 0.5.0

[shadows-withal]

Screenshots:

• Testing with the repo from oranda build: "failed to read asset from" No such file or directory (os error 2) #639
  

• With a nonexistent hardcoded mdbook directory