Improve Kover documentation

[shanshin]

Proposed changes:

• add a detailed description of the Kotlin Kover: purpose, differences, pros and cons.
• explicitly mention example projects
• improve the description of the minimum setup for the ”quick start”
• explain the difference between Kotlin/Groovy code snippets in more detail
• add the requirement to enable Kover in all modules in the description
• merge “Single-project tasks” with “Configuring project” and “Cross-project coverage with Configuring merged reports”
• describe the difference between kover{} and kovermerged{}
• add a Kover HTML report description
  • when reading the report - some of the lines are green and some are red. It’s quite straightforward - they’re either fully tested or not at all. But the yellow ones are a bit unclear
  • explicitly indicate that the report only shows coverage by lines, it cannot provide detailed information on each expression in a line