Improve beginners tutorial

[nikolaiam]

• Fix broken formatting in helm-spacemacs-help
• Add info about fallback fonts
• Explain functioning of menus
• Add a section on accessing files as per Tutorial for new users #773
• Expand section on troubleshooting and further info
• Add link to Eivind Fonn's Spacemacs ABC videos on youtube (is this OK? I think they are good!)
• Embed links

I think this answers most of the issues in #773 except the tutorial is not launched on first startup if .spacemacs doesn't exist.

[d12frosted]

Thank you very much for this update! Usually we ask to squash all commits (or I do it myself), but for this PR it made sense to keep all commits separately.

That being said I cherry-picked your commits to develop, so you can safely delete your branch. In case you wish to further improve documentation, you might find this link helpful. 😸 Just saying 😸 😈

[nikolaiam]

Thanks! The tip is appreciated, I browsed through the issues behind the link and they seem like something I could try to help with :) Although I found some issues with the Beginner's tutorial still, so decided to fix them first!