Added a guide about managing software dependencies

This follows on from a conversation on the GOV.UK developers mailing list.
Minor tweaks

- Style guide
- Spelling
add a technical architect role index
Retag lots of the content for technical architects
minor typos
Fix typos
Return a more useful exit code from linting

We should not ‘compile’ the service manual if there are broken links.
Add usage for lint application
Exclude lint.go from jekyll copy
Fix error when merging
Make elements page show up in search

Only exclude index.md files that have a role-index layout. That layout
means that the page is mainly intended to be a catalogue of links rather
than actual content. A page of links should not show up in our search
results, since that would introduce an extra click to the user journey.

After this change:

* https://www.gov.uk/service-manual/search?q=captcha
  should only have a single result.
  https://www.gov.uk/service-manual/developers should not be in the
  results for that search. This is unchanged behaviour.
* https://www.gov.uk/service-manual/search?q=colour should have
  https://www.gov.uk/service-manual/user-centred-design/resources/elements
  in the results for that search. This is the new, desired behaviour.
Merge pull request #367 from alphagov/index-elements

Make elements page show up in search
Merge pull request #365 from alphagov/tech-arch-role-index

Tech arch role index
Remove unnecessary heading from 'Helpdesk' page
Update subtitle of 'Hosting' page
Rewrite of 'Accessibility testing' for style

bullet lists, removal of banned words, plain English etc.
Rewrite of 'Analytics tools' for style

bullet lists, plain E, removal of banned words...
Rewrite of 'Configuration management' for style

bullet lists, removal of banned words, plain E, voice...
Rewrite of 'Cookies' for style

bullet lists, voice, removing banned words etc
Rewrite of 'Information security' for style

bullet lists, removing banned words, plain E, layout etc