feat: add agenda plugin

Import the package from my dotfiles.

doc/agenda.txt

@@ -0,0 +1,25 @@
+*agenda.txt*
+
+*Agenda*
+
+Neovim integration for the `agenda` <https://git.rkcsd.com/jonas/agenda>
+edit subcommand.
+
+To open an agenda note file: >vim
+
+    Agenda last monday
+
+If the agenda note does not exist, you can create it automatically: >vim
+
+    Agenda! last monday
+
+The date spec can be any string understood by your `date(1)` util.
+
+The command accepts the b, c, e, and t options of the `agenda edit`
+subcommand: >vim
+
+    Agenda [-bc] [-e <extension>] [-t <name>] last monday
+
+See `:Man agenda` for additional information.
+
+vim:tw=78:ts=8:noet:ft=help:norl: