Move docs from wiki to a repository

[php-coder]

Let's move documentation from wiki to the repository.

We'll have the following benefits:

• history of changes
• documentation changes could be submitted as PR
• easy to search locally
• repo become self-sufficient
• documentation can be viewed offline

Checklist:

• [ ] port "How to run unit tests (Java)"
• [ ] port "How to run unit tests (JavaScript)"
• [ ] port "How to run integration tests"
• [x] port "How to check by CheckStyle" (removed because of #1669)
• [x] port "How to check by PMD/CPD" (removed because of #1669)
• [x] port "How to check by FindBugs" (removed because of #1669)
• [x] port "How to check by CodeNarc" (removed because of #1669)
• [x] port "How to calculate code coverage" (removed because of #1672)
• [ ] port "How to check presence of license headers"
• [x] port "How to check ordering of pom.xml" (removed because of #1669)
• [x] port "How to check Robot Framework test cases" (removed because of #1669)
• [x] port "How to generate JavaDoc" (removed because of #1672)
• [ ] add index page with table of content and links to non-existing pages with related issues