Documentation: Don't hardcode install directory

Use the datadir option to select the directory in which to install
documentation. This defaults to $prefix/share so this doesn't introduce
any change in the default case.

While at it use join_paths() to join patch components instead of
hardcoding the / separator.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Documentation/meson.build

@@ -1,3 +1,5 @@
+doc_install_dir = join_paths(get_option('datadir'), 'doc', 'libcamera-@0@'.format(api_version))
+
 sphinx = find_program('sphinx-build-3', required: false)
 if not sphinx.found()
     sphinx = find_program('sphinx-build', required: false)
@@ -16,5 +18,5 @@ if sphinx.found()
                   output: 'html',
                   build_by_default: true,
                   install: true,
-                  install_dir: 'share/doc/libcamera-@0@'.format(api_version))
+                  install_dir: doc_install_dir)
 endif