Doxygen fixes.

The DFU service doc was filtered due to f8ab6daf70
Ignore two MD files which must not appear in Doxygen
Fix Doxygen link

README.md

@@ -16,10 +16,13 @@ Currently supported services include:
 * UriBeacon
 * iBeacon
 
+The [documentation](https://docs.mbed.com/docs/ble-intros/en/latest/AdvSamples/Overview/)
+contains an overview on how to create new, application-specific services.
+
 # Getting Started
 The mbed BLE API is meant to be used in projects on developer.mbed.org. Please see examples and sample project files there.
 A good starting point are these pages:
 * [mbed BLE Homepage](https://developer.mbed.org/teams/Bluetooth-Low-Energy/) for all things BLE
 * [mbed BLE Getting Started Guide](https://developer.mbed.org/forum/team-63-Bluetooth-Low-Energy-community/topic/5262/) a wonderful primer on using BLE with mbed
 * [mbed BLE doc](https://docs.mbed.com/docs/ble-intros/en/latest/) for an introduction to mbed BLE
-* [mbed BLE API page](https://developer.mbed.org/teams/Bluetooth-Low-Energy/code/BLE_API/docs/tip/) for the API in generated by doxygen
+* [mbed BLE API page](https://docs.mbed.com/docs/ble-api/en/latest/api/index.html) for the Doxygen API documentation

ble.doxyfile

@@ -696,7 +696,7 @@ RECURSIVE              = YES
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                = configs
+EXCLUDE                = configs CONTRIBUTING.md README.md
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded
@@ -1611,18 +1611,7 @@ INCLUDE_FILE_PATTERNS  =
 # undefined via #undef or recursively expanded use the := operator
 # instead of the = operator.
 
-PREDEFINED             = WIN32 \
-                         NTLM \
-                         USE_LZO \
-                         ENABLE_FRAGMENT \
-                         P2MP \
-                         P2MP_SERVER \
-                         USE_CRYPTO \
-                         USE_SSL \
-                         ENABLE_PLUGIN \
-                         ENABLE_MANAGEMENT \
-                         ENABLE_OCC \
-                         HAVE_GETTIMEOFDAY
+PREDEFINED             = TARGET_NRF51822
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then
 # this tag can be used to specify a list of macro names that should be expanded.