Fix links and upgrade DocFX to 2.55

docs/BuildDoc.cmd

@@ -1,6 +1,6 @@
 @ECHO OFF
 
-SET DocFXVersion=2.48.1
+SET DocFXVersion=2.55
 SET DocFxFolder=%~dp0..\.tools\docfx
 
 REM Download DocFx

docs/Features.md

@@ -6,7 +6,7 @@
   - Trimble Laser Range Finder: `PTNLA`, `PTNLB`
   - TruePulse Laser Range Finder: `PLTIT`
 - Automatic merging of multi-sentence messages for simplified usage.
-- Extensible with custom NMEA messages. [See here](concepts/CustomMessages.html)
+- Extensible with custom NMEA messages. [See here](concepts/CustomMessages.md)
 - Multiple input devices out of the box
   - System.IO.Stream (all platforms)
   - Emulation from NMEA log file (all platforms)

docs/api/index.md

@@ -2,17 +2,17 @@
 
 **Select your platform:**
 
-- [.NET Standard API Reference](netstd/index.html)
+- [.NET Standard API Reference](netstd/index.md)
 
-- [.NET Framework API Reference](netfx/index.html)
+- [.NET Framework API Reference](netfx/index.md)
 
-- [.NET Core API Reference](netcore/index.html)
+- [.NET Core API Reference](netcore/index.md)
 
-- [UWP API Reference](uwp/index.html)
+- [UWP API Reference](uwp/index.md)
 
-- [Android API Reference](android/index.html)
+- [Android API Reference](android/index.md)
 
-- [iOS API Reference](ios/index.html)
+- [iOS API Reference](ios/index.md)
 
 ### Object model diagrams
 Click below to see the full object model diagram:
@@ -22,4 +22,4 @@ Click below to see the full object model diagram:
 ### Platform specific devices:
 
 Note: The library reference is identical on each platform, except some platforms have platform-specific additional NMEA devices to simplify usage on those platforms. While they may be named the same, they might be have slightly different or take different platform-specific arguments.
-See the menu in the "[Getting Started](../concepts/index.html)" section for more info on these.
+See the menu in the "[Getting Started](../concepts/index.md)" section for more info on these.