Update quality to include exploratory testing

A re-write and update of the exploratory testing section of the manual.
- added link to exploratory testing page in quality.md
- created separate exploratory testing page
- linked to exploratory testing page from testing-in-agile.md
Header table fixed

Meta table in the header was incorrectly formatted
Merge pull request #497 from alphagov/update-quality

Update quality, self merge as nobody else seems to want to touch it...