slugifying produces duplicate `div` ids

In a project that I am converting to documentation I have:
* `gdal.Group.arrays` property in `gdal.Group` class
* `gdal.GroupArrays` class

They both end up having a div id of `gdalgrouparrays` and the fragment links do not work

I propose that the slugifying is tracked:
* we always reuse the same instance of `GithubSlugger`
* every time a new slug is requested, it is checked against a list of existing slugs and only if it does not exist, a new one is requested from `GithubSlugger`