Add release note for FCS bump.

Author: nojaf

RELEASE_NOTES.md

@@ -1,6 +1,6 @@
 # Changelog
 
-## [Unreleased]
+## 20.0.0-alpha-013 - 2023-11-21
 
 ### Added
 * Add more options to customize colors.
@@ -9,6 +9,9 @@
 - `--fsdocs-theme-toggle-light-color` and `--fsdocs-theme-toggle-dark-color` are now deprecated. Use `--header-link-color` instead.
 - `<FsDocsCollectionNameLink>`
 
+### Changed
+- Update FCS to 43.8.100
+
 ## 20.0.0-alpha-012 - 2023-11-17
 
 ### Added