Revamp

General structure, wording and formatting revamp.

Notably, common problems and solutions are extracted into a separate document called "Fix Problems" (./fix-problems.md).

Changes are everywhere so I didn't follow the usual pattern of small commits. Instead, please proofread the new version as a whole, and compare with memory/reread of the old/current version.

In part inspired by #2 (closed), in an attempt to be more clear and friendlier to newcomers. @kaholaz, please proofread and let me know if the reading flow is any better, and what remains unclear ;)