[reference] Add tools.files.chmod description and reference

[uilianries]

Related to conan-io/conan#17800

It requires Conan develop2 branch to be generated.

Preview - Docs commit 2985991 + Conan commit 63e193b

[czoido]

If we accept None as value for read, write and execute, shouldn't the type hints be as follows?

read: Optional[bool] = None,
write: Optional[bool] = None,
execute: Optional[bool] = None,

Also, I think that it should be documented in the docstring that True will add the permission, False will remove it and the default that is None does not modify it.