rfctr(html): drop page concept (#3184)
**Summary**
Pagination of HTML documents is currently unused. The `Page` class and
concept were deeply embedding in the legacy organization of HTML
partitioning code due to the legacy `Document` (= pages of elements)
domain model. Remove this concept from the code such that elements are
available directly from the partitioner.
**Additional Context**
- Pagination can be re-added later if we decide we want it again. A
re-implementation would be much simpler and much lower impact to the
structure of the code and introduce much less additional complexity,
similar to the approach we take in `partition_docx()`.
