WIP: Migrate Wiki content over to src/docs

docs/gn_check.md

+GN has several different ways to check dependencies. Many of them are checked by the `gn check` command. Running checks involve opening and scanning all source files so this isn't run every time a build is updated. To run check on an existing build:
+```
+gn check out/mybuild
+```
+
+To run the check as part of the "gen" command to update the build (this is what the bots do):
+```
+gn gen out/mybuild --check
+```
+
+# Concepts
+
+## Visibility
+
+Targets can control which other targets may depend on them by specifying `visibility`. Visibility is always checked when running any GN command (not just `gn check`.
+
+By default, targets are "public" meaning any target can depend on them. If you supply a list, visibility will be listed to those targets (possibly including wildcards):
+
+```
+visibility = [
+  ":*",  # All targets in this file.
+  "//content/*",  # All targets in content and any subdirectory thereof.
+  "//tools:doom_melon",  # This specific target.
+]
+```
+
+See `gn help visibility` for more details and examples.
+
+## Public header files
+
+Targets can control which headers may be included by dependent targets so as to define a public API. If your target specifies only `sources`, then all headers listed there are public and can be included by all dependents.
+
+If your target defines a `public` variable, only the files listed in that list will be public. Files in `sources` but not `public` (they can be in both or only one) may not be included by dependent targets.
+
+```
+source_set("foo") {
+  public = [
+    "foo.h",
+    "foo_config.h",
+  ]
+  sources = [
+    "foo.cc",
+    "foo.h",
+    "bar.cc",
+    "bar.h",
+  ]
+}
+```
+
+## Public dependencies
+
+In order to include files from your target, that target must be listed in your target's dependencies. By default, transitively depending on a target doesn't give your files this privilege.
+
+If a target exposes a dependency as part of its public API, then it can list that dependency as a `public_deps`:
+```
+source_set("foo") {
+  sources = [ ... ]
+  public_deps = [
+    "//base",
+  ]
+  deps = [
+    "//tools/doom_melon",
+  ]
+}
+```
+Targets that depend on `foo` can include files from `base` but not from `doom_melon`. To include public headers from `doom\_melon, a target would need to depend directly on it.
+
+Public dependencies work transitively, so listing a target as a public dependency also exposes that target's public dependencies. Along with the ability to include headers, public dependencies forward the `public_configs` which allow settings like defines and include directories to apply to dependents.
+
+# Putting it all together
+
+In order to include a header from target Y in a file that is part of target X:
+
+  * X must be in Y's `visibility` list (or B must have no `visibility` defined).
+  * The header must be in Y's `public` headers (or Y must have no `public` variable defined).
+  * X must depend directly on Y, or there must be a path from X to Y following only public dependencies.
+
+### What gets checked
+
+Chrome currently doesn't come close to passing a `gn check` pass. You can check specific targets or subtrees for issues:
+```
+  gn check out/mybuild //base
+
+  gn check out/mybuild "//mojo/*"
+```