Rename "rendering" to "graphics"

This makes the section more obvious, and the page itself an overview of the graphics
at a high level—the *rendering* process will be described separately in one of the
upcoming commits.

Author: ISSOtm

src/Rendering.md renamed to src/Graphics.md

@@ -1,4 +1,4 @@
-# Rendering Overview
+# Graphics Overview
 
 The Game Boy outputs graphics to a 160×144 pixel LCD, using a quite complex
 mechanism to facilitate rendering.

src/SUMMARY.md

@@ -15,7 +15,7 @@
 # I/O Ports
 
 - [Summary](./Hardware_Reg_List.md)
-- [Rendering](./Rendering.md)
+- [Graphics](./Graphics.md)
   - [Tile Data](./Tile_Data.md)
   - [Tile Maps](./Tile_Maps.md)
   - [OAM](./OAM.md)