Update docs for v1.9

Description

Closes issues: - Related issues: LA-93

Includes docs for MRs !341 (merged), !346 (merged), !347 (merged), !348 (merged), !349 (merged), !350 (merged), !354 (merged), !355 (merged), !356 (merged), !358 (merged), !359 (merged) and !360 (merged). Simplified docs by removing "Concepts" as a nav item, moved most contents to "User manual" (changed from "User interface") except filtering, which has been merged with the corresponding doc in /technical.

Notes to reviewer

@oyvinev should review changes in /technical in particular (related to !349 (merged)) Done. Hold off on merge until release of v1.9 is ready in case of last minute changes.

Type of change

Application (affects UI or general functionality):

• New feature
• Bug fix
• Improvement

Ops / admin / CI related only (not impacting users):

• New feature
• Bug fix
• Improvement

Tests

General

• Tests have been added that prove my fix is effective or that my feature works
• Related tests have been modified/removed

Hypothesis testing:

• Soak testing has been done
• Distribution between positive / negative cases has been checked

Database

• Includes changes to database schema
• Includes necessary database migrations

Configuration

• Includes changes to configuration
• Includes configuration migration instructions in documentation

Merge checklist

• Self-review of code performed
• Feature review against specification (if applicable)
• Need for documentation has been evaluated and, if necessary, updated
• Code and implementation is reviewed by other core developer (all changes, inc. changes based on initial review)