Add more standard ignored files for Julia (github#2992)

* Add more standard ignored files for Julia

In particular, this adds documentation build artifacts generated by Documenter.jl as well as Manifest.toml, which can appear in docs/, in test/, or at the top level.

* Clarify the intent of each ignored item

Also add a few more build artifacts from BinaryProvider/BinDeps.

Julia.gitignore

@@ -1,4 +1,24 @@
+# Files generated by invoking Julia with --code-coverage
 *.jl.cov
 *.jl.*.cov
+
+# Files generated by invoking Julia with --track-allocation
 *.jl.mem
+
+# System-specific files and directories generated by the BinaryProvider and BinDeps packages
+# They contain absolute paths specific to the host computer, and so should not be committed
 deps/deps.jl
+deps/build.log
+deps/downloads/
+deps/usr/
+deps/src/
+
+# Build artifacts for creating documentation generated by the Documenter package
+docs/build/
+docs/site/
+
+# File generated by Pkg, the package manager, based on a corresponding Project.toml
+# It records a fixed state of all packages used by the project. As such, it should not be
+# committed for packages, but should be committed for applications that require a static
+# environment.
+Manifest.toml