add updating and troubleshooting instructions, remove hosting

[fpvandoorn]

• Add detailed update instructions
• The hosting section was mostly there to link to ci.html which was removed.
• Add troubleshooting instructions when a corrupted Lean version was downloaded
• Add instructions to clean-up old Lean versions/Mathlib cache.

fixes #489

[fpvandoorn]

I now mention lake exe mk_all (without alternatives) and moved lake -R -Kenv=dev update into a parenthetical.