Make timerange more prominent in docs

[samdbmg]

Details

Moves the definition of TimeRange and Timestamp into a JSON schema file, and references it in all the types that use it.
Also adds links from the descriptions in rendered docs.

Pivotal Story

Story URL: https://www.pivotaltracker.com/story/show/186709535

Related PRs

Follows #8

Links to external test runs/working deployment

N/A

Submitter PR Checks

(tick as appropriate)

• Added bbc/rd-apmm-cloudfit team as a reviewer
• PR completes task/fixes bug
• Tests exercise code appropriately
• New features and API breaks are flagged in commit messages using magic strings
• Repo maintainer is notified that a release is required
• Documentation updated (README, Confluence, Docstrings, API spec, Engineering Guide, etc.)
• Downstream repos have been checked for potential breaks & fixed as needed
• APIs/UIs/CLIs updated as required
• PR added to Pivotal story
• Follow-up stories added to Pivotal
• Any temporary code/configuration removed (e.g. test deployment environment, temporary commontooling branch)
• Any pins against pre-releases have been removed

Reviewer PR Checks

(tick as appropriate)

• PR completes task/fixes bug
• Tests exercise code appropriately
• Design makes sense, and fits with our current code base
• Code is easy to follow
• PR size is sensible
• Commit history is sensible and tidy

Info on Cloudfit PRs

• The checks above are guidelines. They don't all have to be ticked, but they should all have been considered.
• For more details on how to asses PRs, see: https://github.com/bbc/rd-apmm-docs-ways-of-working/blob/master/workflow/PRChecklist.md

[j616]

LGTM