docs: Various updates for 2.3, 2.4 beta, and Go 1.16

Steps on #139 (sorry)

src/docs/markdown/caddyfile/directives/abort.md

@@ -0,0 +1,22 @@
+---
+title: abort (Caddyfile directive)
+---
+
+# abort
+
+Prevents any response to the client by immediately aborting the HTTP handler chain and closing the connection. Any concurrent, active HTTP streams on the same connection are interrupted.
+
+
+## Syntax
+
+```caddy-d
+abort [<matcher>]
+```
+
+## Examples
+
+Abort all requests for paths starting with `/foo`:
+
+```caddy-d
+abort /foo*
+```