doc: add the tool to check section marks in help documents This patch adds "doc/check-seclevel.py" which checks below in help documents: - whether unknown or unavailable section marks are used or not - whether appropriate section mark is used at sub-sectioning It should be invoked in "doc" directory. It checks all help documents of Mercurial (topics, commands, extensions), if no file is specified by --file option. With --file option, it checks contents of the specified file as help document, for self testing purpose: -t/-c/-e/-C are used to specify what kind of help document contents of the specified file is. This checking is related to changeset 979b107eaea2.