Introduction section is missing from the user guide. Also report hashes section is missing.

[dkrupp]

Describe the bug The user guide should have an intro section explaining and summarizing the Features of the CodeChecker command and its subcommands (each one sentences).

The user guide should also have a separate section on Report hashes, because it is a particularly important topic. Now it is hidden in the help of the check command https://github.com/Ericsson/codechecker/blob/master/docs/analyzer/user_guide.md#check-