docs: add documentation for building with meson

docs: add documentation for building with meson

v2: - Add information about CC, CXX, CFLAGS, and CXXFLAGS (Nicolai)
    - Add message at top that meson for mesa is still a work in progress
    - Add trailing "/" to directories (Eric E.)
    - Fix a number of spelling/grammar/style suggestions from Eric E.
    - Make a number of changes as suggested by Emil.
v3: - Fix order of commands in example (Eric E.)
    - Add documentation for overriding LLVM version (Eric E.)
v4: - Rebase on master
    - update default buildtype
    - add note about b_ndebug
    - Clarify meson configure a bit
v5: - use <code> for command line arguments (Eric E.)
    - Add note about listing options without a build directory
    - Minor formatting changes (Eric E.)
    - Replace the CC, CFLAGS, etc section with an environment variables
      section, which mentions CC, CXX, CFLAGS, CXXFLAGS, LDFLAGS, and
      DESTDIR
    - Add comment that not using buildtype debug might make debugging
      harder
    - Add comment that b_ndebug and buildtype are orthogonal

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch> (v3)