doc: add Build Python from Source wiki page

doc/wiki/Build-Python-from-Source.md

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+Building Python from source takes very little time and effort!  Just follow
+these easy steps:
+
+### Clone the source repository
+
+Make a shallow clone at the tip of the desired branch, `3.15` in this example.
+To build a specific version, you’d specify a tag instead a branch, e.g.
+`--branch=v3.14.7`.
+
+```bash
+$ git clone --depth=1 --branch=3.15 https://github.com/python/cpython.git
+```
+
+### Configure, compile, and install
+
+In this example, we’ll build a free-threaded interpreter. For a single-threaded
+one you’d omit the `--disable-gil` option.
+
+Omitting `--enable-optimizations` will also cause the tests to be skipped and
+thus greatly speed up the build, though this not recommended.  Compilation
+without the tests typically takes around 10 seconds (!) on a modern 12-core
+computer.
+
+```bash
+$ cd cpython
+$ ./configure --enable-optimizations --disable-gil --prefix=$HOME/py315-nogil
+$ make -j
+$ make install
+```
+
+### Update execution path
+
+Your Python 3.15 binaries are now in `~/py315-nogil/bin`.  Add the directory to
+your path and check the installation:
+
+```bash
+$ export PATH=$HOME/py315-nogil/bin:$PATH
+$ python3 --version
+$ python3 -m pip --version
+```
+
+That’s all!  Your custom Python installation is now ready to use!