SmoothieV2 new-feature documentation

Reconcile the SmoothieV2 New features wiki and Maker Forums announcement with current firmware source and existing documentation.

Document V2 button-box FAULT inputs and the matrix-keypad implementation status.

Document V2 auxiliary UART console/target modes and echo -1 usage.

Correct and expand the V2 NIST G30/G30.1 documentation.

Document the ed file editor and le interactive line editor with command history.

Document V2 network shell, FTP, HTTP/WebSocket, and NTP services.

Document MAX7219 DRO configuration and operation.

Document SmoothieV2 O-word subroutines.

Verify and complete lathe/ELS, slaved-axis, and direct-MPG coverage.

Document instant feed hold/cycle start and experimental V1 backlash compensation from the forum announcement.

Add direct homepage and relevant-section links for every feature in the source inventories.

Review new prose for site style, source accuracy, limits, and edge cases.

Run documentation tests and an isolated low-priority Jekyll build.

Verify pages and navigation in a separate browser preview. (CRITICAL: Don’t use main dev server on port 4000. Set PORT=4187, run separate server.)