🔨 Clean up build scripts (#22264)

* Add 10K to marlin_blackSTM32F407VET6 (typo?)
* Document custom build scripts.
* Add a Robin common build script.
* Extraneous .ldscript specifiers

buildroot/share/PlatformIO/boards/marlin_blackSTM32F407VET6.json

@@ -48,7 +48,7 @@
   "upload": {
     "disable_flushing": false,
     "maximum_ram_size": 131072,
-    "maximum_size": 514288,
+    "maximum_size": 524288,
     "protocol": "stlink",
     "protocols": [
       "stlink",