Provide more cross links for the documentation, FAQs and READMEs

[PhilipOakley]

It's not always obvious where the various documentation parts are located, especially if one arrives at the 'wrong' web site (like here..).

Please can a wiki page be added linking to the https://www.sourcetrail.com/documentation/#PROJECTSETUP and any others I have missed.

The YouTube videos could also be linked e.g. https://youtu.be/7fguPwKR_7Y

Also add similar links in the README, etc.

It is easy to give up if one can't find the docs.