meson: build html-docs

This allows us to build our Sphinx-based documentation from the meson
build system.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11494>

docs/meson.build

@@ -0,0 +1,15 @@
+html_docs = custom_target(
+  'html',
+  depend_files: files('conf.py'),
+  command: [sphinx, '-b', 'html', '-q', meson.current_source_dir(),
+            '@OUTPUT@'],
+  output: 'html',
+  build_by_default: get_option('html-docs').enabled(),
+)
+
+install_subdir(html_docs.full_path(),
+  install_dir: join_paths(get_option('datadir'), 'doc', 'mesa'),
+  exclude_files: ['.buildinfo'],
+  exclude_directories: ['.doctrees'],
+  strip_directory: true
+)