Update for Documenter.jl v1 (#683)

* ignore `docs/Manifest.toml`

* add missing docstrings

* add compat table for Documneter.jl

* rename Other API with Internal API

* Use `checkdocs=:exports` and ignore internal docstrings as strongly suggested by the reviewer

.gitignore

@@ -4,5 +4,6 @@
 *.DS_Store
 /docs/build/
 /docs/site/
+/docs/Manifest.toml
 /benchmark_data/
 /Manifest.toml

docs/Project.toml

@@ -1,3 +1,6 @@
 [deps]
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 ForwardDiff = "f6369f11-7733-5829-9624-2563aa707210"
+
+[compat]
+Documenter = "1"

docs/make.jl

@@ -11,7 +11,8 @@ makedocs(modules=[ForwardDiff],
                     "Upgrading from Older Versions" => "user/upgrade.md"],
                   "Developer Documentation" => [
                     "How ForwardDiff Works" => "dev/how_it_works.md",
-                    "How to Contribute" => "dev/contributing.md"]])
+                    "How to Contribute" => "dev/contributing.md"]],
+         checkdocs=:exports)
 
 deploydocs(
     repo = "github.com/JuliaDiff/ForwardDiff.jl.git"