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
Break up long sentence
dash pedantry
style guide sweep
Added sentence

Requested by Deputy CTO
Added full stop
Merge pull request #425 from alphagov/agile-formatting

Minor cleanup
Merge pull request #427 from alphagov/culture-change-update-

Added sentence
Fix typo highlighted by @waynemoir

Fixes #429.
Remove numbers
Remove FAQ format
Simplify and apply style guide for lists
Reduce headings
Add punctuation
Merge pull request #430 from alphagov/feature/agile-training-tweaks

Style changes for new agile training content
Merge pull request #428 from alphagov/add-sinatra-app

Add a Sinatra app