Docs updates for the 0.15 release

[radinamatic]

Summary of changes:

• added instructions and screenshots for the new features in 0.15
• updated the icons for all the new learning activities. Fixes #219.
• removed the hardcoded version mentions and replaced with substitution docstring. Fixes #68.
• added warning for permission problems during export (mentioned in https://github.com/learningequality/kolibri/issues/6685#issuecomment-603283760).
• left only Catalina (10.15) macOS compatibility, as that is the only version where we've been able to run the full critical workflows test plan. If we receive the confirmation that it runs without issues on more recent versions, we'll update. Fixes #237.
• changed all headings from Title Case to Sentence case.