slugifying produces duplicate `div` ids

[mmomtchev]

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