Add "Choosing appropriate formats"

After some discussion on govuk-dev about publication formats, I said I'd attempt
to draft something for the service manual. This is very rough and ready, but
hopefully better than nothing.
added accessible formats
Create creating-accessible-PDFs.md
added link
corrected link
link to Nick Stenning's piece
Use .html not .md suffixes for links to content
Link to accessible PDF guidance
Add YAML front matter to accessibility guidance

Allows page to function correctly
Fix copy in accessible PDF guidance
Merge pull request #42 from alphagov/formats

Add "Choosing appropriate formats"