v3: Add docs for clear_buildpack_cache endpoint

[Finishes #163341383]

Co-authored-by: Belinda Liu <bliu@pivotal.io>
Co-authored-by: Reid Mitchell <rmitchell@pivotal.io>

Author: belinda-liu

docs/v3/source/includes/resources/admin/_clear_buildpack_cache.md.erb

@@ -0,0 +1,34 @@
+### Clear buildpack cache
+
+```
+Example Request
+```
+
+```shell
+curl "https://api.example.org/v3/admin/actions/clear_buildpack_cache" \
+  -X POST \
+  -H "Authorization: bearer [token]"
+```
+
+```
+Example Response
+```
+
+```http
+HTTP/1.1 202 Accepted
+Location: https://api.example.org/v3/jobs/[guid]
+```
+
+This endpoint will delete all of the existing buildpack caches in the
+blobstore. The buildpack cache is used during staging by buildpacks as a way to
+cache certain resources, e.g. downloaded Ruby gems. An admin who wants to
+decrease the size of their blobstore could use this endpoint to delete
+unnecessary blobs.
+
+#### Definition
+`POST /v3/admin/actions/clear_buildpack_cache`
+
+#### Permitted Roles
+ |
+--- | ---
+Admin |

docs/v3/source/includes/resources/admin/_header.md.erb

@@ -0,0 +1,3 @@
+## Admin
+
+These endpoints are only for admin users.

docs/v3/source/index.md

@@ -55,6 +55,8 @@ includes:
   - concepts/status_codes
   - concepts/warnings
   - resources/header
+  - resources/admin/header
+  - resources/admin/clear_buildpack_cache
   - resources/apps/header
   - resources/apps/object
   - resources/apps/create