Add link to Ashby instructions to Hiring page in Handbook

[charlescook-ph]

Changes

FYI @couaphang @postgrace we now have full Ashby instructions (thank you @smallbrownbike !)

Checklist

• [ ] Titles are in sentence case
• [ ] Feature names are in sentence case too
• [ ] Words are spelled using American English
• [ ] I have checked out our style guide