H2 slugs changed to make anchor text linking logical

slugs are now:
- the name of the numbered section that H2 appears in, followed by
- all the words that appear in the title (ignoring all punctuation)

Example:
news-stories-titles
policy-pages-titles
publications-titles
and
policy-pages-who-were-consulting-or-who-weve-consulted-optional-and-depending-on-time
Update detailed-guides.html.erb
Merge pull request #55 from ElenaFdR/patch-1

H2 slugs changed to make anchor text linking logical
Merge pull request #54 from ElenaFdR/patch-2

Updated H2 headings so that all H2s can be linked to
Merge pull request #57 from alphagov/correct-subheading-ids

Correct subheading ids