Epic: improve infrastructure documentation

[ChristianKuehnel]

This issue is a collection of all the documentation improvements we discussed:

• [ ] Update links to code coverage tools #13
• [x] Document how to find owners of BuildBots https://github.com/ChristianKuehnel/iwg-workspace/issues/37
• [x] Document generation of the website #17
• [ ] Document test strategy #18
• [ ] Upload tutorials to LLVM website https://github.com/ChristianKuehnel/iwg-workspace/issues/13
• [ ] scan through list of BuildBot workers, check if owner information is available #20
• [ ] Document which sub-projects are using Phabricator and PullRequests #21
• [x] Document pre-merge checks https://github.com/google/llvm-premerge-checks/issues/275

[ChristianKuehnel]

stale effort, removing from out backlog