TTYG-178 Improve documentation

[pgan002]

Changes

• Move most of the documentation from the README file into separate files in a new directory docs/. Benefits:
  1. The main page (README) is shorter, and so more welcoming
  2. The main page loads faster
  3. The sections are shorter and so easier to read
  4. The directory helps to understand the contents
• Major additions for completeness
• Major edits clarity
• Links to documentation sections from README and from other sections
• Consistent section heading case: sentence case
• Join each paragraph into a single line for easier MD editing

Tests

• Spelling, grammar, typos: copy-pasted text into word processor
• Links: manually followed each link in README.md and docs/*

Open questions

• How to format the key-definitions in § Output: as a list or table?
• Move section "Aggregate metrics" to a separate file or into section Metrics?