cache element indexx

[yuval-peleg-levy-ai21]

Hey,
I have a project in which I insert a lot of HTML boxes. I noticed that it is very slow, and after profiling figured
doc.get_page_images, doc.get_page_xobjects, doc.get_page_fonts are to blame.

Therefore, I'd like to propose maintaining a counter in the Page class to save those calls.
In my local machine it saved 90% of the compute time.

This PR replaces #4786

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