doc/main/CHANGELOG: use keepachangelog.com recommendations

This updates the changelog to use keepachangelog.com recommendations
to make it consistent with other Pybricks changelogs.

doc/main/CHANGELOG.md

@@ -1,66 +1,70 @@
-# 1.6.0 - 2021-08-30
+# Changelog
 
-## Added:
+<!-- refer to https://keepachangelog.com/en/1.0.0/ for guidance -->
+
+## 1.6.0 - 2021-08-30
+
+### Added
 - MicroPython module documentation.
 - Examples for hub system functions including stop button and shutdown.
 
-## Changed:
+### Changed
 - Build IDE docs as main docs with minor changes, instead of a completely
   separate build.
 - Moved motor control documentation to the motor page.
 
-# 1.5.0 - 2021-07-01
+## 1.5.0 - 2021-07-01
 
-## Added:
+### Added
 - Documentation for Powered Up Remote Control.
 
-# 1.4.0 - 2021-06-23
+## 1.4.0 - 2021-06-23
 
-## Added:
+### Added
 - Enabled beta content that was hidden for the 3.0 release.
 - Added notice about using the latest beta version.
 
-# 1.3.3 - 2021-05-21
+## 1.3.3 - 2021-05-21
 
-## Changed:
+### Changed
 - Match example snippet styling to IDE.
 - Add more examples.
 
-# 1.3.2 - 2021-04-26
+## 1.3.2 - 2021-04-26
 
-## Changed:
+## Changed
 - Theme style fixes.
 - Example code fixes
 - Match doc version to firmware version.
 
-# 1.3.1 - 2021-04-12
+## 1.3.1 - 2021-04-12
 
-## Changed:
+### Changed
 - Upgrade sphinx and rtd-theme to fix style issues.
 
-# 1.3.0 - 2021-04-12
+## 1.3.0 - 2021-04-12
 
-## Removed:
+### Removed
 - Removed features which not be in the official 3.0 release. These features
   are still in beta. They'll come back in future releases once tested.
 
-# 1.2.0 - 2021-04-09
+## 1.2.0 - 2021-04-09
 
-## Changed:
+### Changed
 - Moved installation guide to external site.
 
-# 1.1.1 - 2021-02-14
+## 1.1.1 - 2021-02-14
 
-## Added:
+### Added
 - Added installation guide.
 - Various documentation fixes.
 
-# 1.1.0 - 2021-01-28
+## 1.1.0 - 2021-01-28
 
-## Added:
+### Added
 - Scrollbar styling.
 
-# 1.0.0 - 2021-01-25
+## 1.0.0 - 2021-01-25
 
-## Added:
+### Added
 - Sphinx build output.