covid-19-services: Add docs about the new find-support form
covid-19-services: Add docs about the new find-support form

- This is WIP as there are various TODOs: start page, Sentry links.
- Where relevant I changed "both" to "all" or got rid of the word
  entirely as there are now >2 forms.
- Fix some casing of products, like "PaaS" and fix some accidental,
  unnecessary double spacing between sections.
- Move the blurb about the IVR to the vulnerable people section, as
  that's the only thing it's for.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs