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.