Updated API Changes (markdown)
Added SC 5.21 changelog.
SC 5.18 release date
Added SC 5.18 changelog to the list.
Added SC 5.17 release date.
Added changelog for SC 5.17.
Added SC 5.16 changelog to the list.
Added changelog for SC 5.15.
Formatting.
Formatting. Added AS versions.
Added old changelogs and release dates
Added entry for the 5.13 changelog.
Added SC 5.12 API changes.
Removed "(upcoming)" from the 5.11 link.
Added link to 5.09 API changelog.
Added "API Changes" pages.