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 Update rst icons once Learning Activities icons are in KDS #219.
• removed the hardcoded version mentions and replaced with substitution docstring. Fixes Add Kolibri version as a substitution string and replace all hard-coded versions #68.
• added warning for permission problems during export (mentioned in Question: workflow for content export kolibri#6685 (comment)).
• 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 update macOS compatibility #237.
• changed all headings from Title Case to Sentence case.