Copyedit and update "Browsers and devices"

- Minor style guide edits
- Rewrite first paragraph, remove unnecessary heading
- Remove all the links to GOV.UK
- Update statistics from StatCounter
- Link to Performance Platform GOV.UK dashboard
- Remove 'Understanding progressive enhancement' link (it's in the relevant guidance)
Update supported browser table

Using data from @edds' browser matrix, focusing specifically
on small screens:

- Add iOS 7 Google Chrome (it's approaching 1%)
- Add Android Browser 4.x (it's also around 1%)
- Add Windows Phone 8 IE 10 as functional (it's about 0.5%)
- Up BlackBerry requirement from 6 to 7.1, make it functional (0.2%)
Change browser tables to use rowspan attribute

To make them more accessible. Hat tip to @bradleywright.

https://github.com/alphagov/government-service-design-manual/pull/340#discussion_r11842934