[DO NOT MERGE] feat(docs): $TURBO_DEFAULT$ (#7155)

### Description

Docs for #7113


Depends on #7113

Closes TURBO-2184

Author: tknickman

docs/pages/repo/docs/core-concepts/caching.mdx

@@ -130,7 +130,7 @@ Now, make sure that `turbo` is aware of the file by adding it to task inputs. Yo
   "pipelines": {
     "build-for-platforms": {
       "dependsOn": ["^build"],
-      "inputs": ["turbo-cache-key.json", ...]
+      "inputs": ["$TURBO_DEFAULT$", "turbo-cache-key.json"]
     }
   }
 }

docs/pages/repo/docs/core-concepts/caching/file-inputs.mdx

@@ -71,6 +71,10 @@ You should aim to have both Git and the Git index of your repository available f
 
 <Callout>
   Customizing the `inputs` immediately opts out of the default behavior. You must manually specify via `pipeline.<task>.inputs` all of the files that should be considered.
+
+For convienence, you can use the special string `$TURBO_DEFAULT$` within an inputs array to automatically include all files considered in the default behavior. This allows
+you to customize the inputs without needing to manually specify all of the files that would have been considered by default.
+
 </Callout>
 
 For some tasks it is worthwhile to reduce the number of file inputs into the task hash consideration. Very few tasks depend on the contents of `README.md`. Specifying just the files that matter can increase the frequency of cache hits.
@@ -114,3 +118,16 @@ If a task depends on another task which specifies file dependencies they do not
 ```
 
 This pattern can be used to save yourself from needing to repetitively enumerate a list of file globs for every single task.
+
+You may also want to keep the default behavior, but only include, or exclude a few files. You can use the special string `$TURBO_DEFAULT$` to include all of the files that would have been considered by default:
+
+```jsonc filename="/turbo.json"
+{
+  "$schema": "https://turbo.build/schema.json",
+  "pipeline": {
+    "build": {
+      "inputs": ["$TURBO_DEFAULT$", "!README.md", "a-git-ignored-file.txt"]
+    }
+  }
+}
+```

docs/pages/repo/docs/reference/configuration.mdx

@@ -361,7 +361,12 @@ Defaults to `true`. Whether or not to cache the task [`outputs`](#outputs). Sett
 
 Tells `turbo` the set of files to consider when determining if a package has changed for a particular task.
 Setting this to a list of globs will cause the task to only be run when files matching those globs have
-changed. This can be helpful if you want to, for example, skip running tests unless a source file changed.
+changed. This can be helpful if you want to, for example, skip running tests unless a source file changed, or
+explicitly depend on a `.gitignore`d file.
+
+Because specifying an `inputs` key immediately opts out of the default behavior, you may specify
+the special string `$TURBO_DEFAULT$` within an inputs array to include all default inputs. This is useful
+if you want to add or remove additional inputs from the default set.
 
 The default is `[]` to run the task when any file in the package changes.
 
@@ -384,6 +389,20 @@ The default is `[]` to run the task when any file in the package changes.
 }
 ```
 
+`$TURBO_DEFAULT$` **Example**
+
+```jsonc
+{
+  "$schema": "https://turbo.build/schema.json",
+  "pipeline": {
+    "check-types": {
+      // Consider all default inputs except the README.md
+      "inputs": ["$TURBO_DEFAULT$", "!README.md"],
+    }
+  }
+}
+```
+
 <Callout type="info">
   Note: `turbo.json` is *always* considered an input. If you modify
   `turbo.json`, all caches are invalidated.