docs: document API for retreiving Klipper "docs"

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

README.md

@@ -20,4 +20,4 @@ Users:\
 [user_changes.md](/docs/user_changes.md) - November 19th 2020
 
 Developers:\
-[api_changes.md](/docs/api_changes.md) - November 19th 2020
+[api_changes.md](/docs/api_changes.md) - November 23rd 2020

docs/api_changes.md

@@ -1,6 +1,10 @@
 This document keeps a record of all changes to Moonraker's remote
 facing APIs.
 
+### November 23rd 2020
+- Moonraker now serves Klipper's "docs" directory.  This can be access
+  at `GET /server/files/docs/<filename>`.
+
 ### November 19th 2020
 - The path for the power APIs has changed from `gpio_power` to `device_power`:
   - `GET /machine/device_power/devices`\

docs/web_api.md

@@ -422,6 +422,7 @@ as the "gcodes" root.  The following roots are available:
 - gcodes
 - config
 - config_examples (read-only)
+- docs (read-only)
 
 Write operations (upload, delete, make directory, remove directory) are
 only available on the `gcodes` and config roots.  Note that the `config` root
@@ -722,6 +723,16 @@ to delete a file in a subdirectory.
 - Returns:\
   The requested file
 
+### Download Klipper documentation
+- HTTP command:\
+  `GET /server/files/docs/<file_name>`
+
+- Websocket command:\
+  Not Available
+
+- Returns:\
+  The requested file
+
 ### Download klippy.log
 - HTTP command:\
   `GET /server/files/klippy.log`