Add search

[yndajas]

The accessibility manual was initially based on the playbook, but the current search feature was added to the playbook after development on the manual started. We should add the search feature to the manual to make it easier for people to find the content they're looking for quickly

Relevant PRs:

• Add search functionality playbook-2015#984
• Add search: iterations playbook-2015#989
• Add search refactor playbook-2015#991
• Remove search from related pages playbook-2015#1034
• Optimise search excerpt to highlight more relevant results playbook-2015#1075