Docs improvements (#375)

* Docs improvements

* Tweak docs deps

* Tweak docs

* Bump patch

* Fix MeanFunction docstring display

Author: willtebbutt

Project.toml

@@ -1,7 +1,7 @@
 name = "AbstractGPs"
 uuid = "99985d1d-32ba-4be9-9821-2ec096f28918"
 authors = ["JuliaGaussianProcesses Team"]
-version = "0.5.17"
+version = "0.5.18"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"