New agile guidance

- Has been style checked
- Need second pair of eyes on markdown and layout
Fixes to agile 'Training and learning'

- Fix breadcrumbs to show location of document
- Fix Markdown: extra newline before heading
- Improve link text (for users and screen readers) - link text should
  reflect what the target page is providing
Merge pull request #420 from alphagov/agile-training

New agile guidance