Repoint B2 refs to new non-boostorg home. (boostorg#570)

Also, relabel Boost.Build to the current B2 name.

CONTRIBUTING.md

@@ -17,7 +17,7 @@ please follow the workflow explained in this document.
   - [5. Update your pull request](#5-update-your-pull-request)
 - [Development](#development)
   - [Install dependencies](#install-dependencies)
-  - [Using Boost.Build](#using-boostbuild)
+  - [Using B2](#using-boostbuild)
   - [Using CMake](#using-cmake)
   - [Running clang-tidy](#running-clang-tidy)
 - [Guidelines](#guidelines)
@@ -35,7 +35,7 @@ please follow the workflow explained in this document.
   it may be a good idea to skim through the
   [Boost Getting Started](https://www.boost.org/more/getting_started/index.html)
   chapters, especially if you are going to use
-  [Boost.Build](https://boostorg.github.io/build/) for the first time.
+  [B2](https://www.bfgroup.xyz/b2/) for the first time.
 
 ## Pull Requests
 
@@ -124,7 +124,7 @@ The preparation involves the following steps:
     git submodule update --init --recursive --jobs 8
     ```
 
-3. Build the `b2` driver program for Boost.Build engine.
+3. Build the `b2` driver program for B2 engine.
 
     ```shell
     ./bootstrap.sh
@@ -311,15 +311,15 @@ Boost.GIL is a [header-only library](https://en.wikipedia.org/wiki/Header-only)
 which does not require sources compilation. Only test runners and
 [example](example/README.md) programs have to be compiled.
 
-By default, Boost.GIL uses Boost.Build to build all the executables.
+By default, Boost.GIL uses B2 to build all the executables.
 
 We also provide configuration for two alternative build systems:
 
 - [CMake](https://cmake.org)
 
 **NOTE:** The CMake is optional and the corresponding build configurations
-for Boost.GIL do not offer equivalents for all Boost.Build features.
-Most important difference to recognise is that Boost.Build will automatically
+for Boost.GIL do not offer equivalents for all B2 features.
+Most important difference to recognise is that B2 will automatically
 build any other Boost libraries required by Boost.GIL as dependencies.
 
 ### Install dependencies
@@ -333,9 +333,9 @@ sudo apt-get install libjpeg-dev libpng-dev libtiff5-dev libraw-dev
 
 **TIP:** On Windows, use vcpkg with `user-config.jam` configuration provided in [example/b2/user-config-windows-vcpkg.jam](example/b2/).
 
-### Using Boost.Build
+### Using B2
 
-The [b2 invocation](https://boostorg.github.io/build/manual/develop/index.html#bbv2.overview.invocation)
+The [b2 invocation](https://www.bfgroup.xyz/b2/manual/release/index.html#bbv2.overview.invocation)
 explains available options like `toolset`, `variant` and others.
 
 Simply, just execute `b2` to run all tests built using default