Update research section structure

Rationalising content, removing redundant pages and simplifying the User research index page. This work is based on research by @Ohrworm . There are some deletions and moving of pages, so ideally these redirects would be put in place:

| Before | After |
|----------|--------|
| /service-manual/making-software/accessibility-testing.html	| /service-manual/user-centred-design/user-research/accessibility-testing.html |
| /service-manual/user-centred-design/know-your-users.html | 	/service-manual/user-centred-design/user-needs.html |
| /service-manual/user-centred-design/introduction-to-user-research.html |	/service-manual/user-centred-design/user-research/index.html|