DOC: Adding header to the rst files that don't have it
When using the new style for the pandas documentation, we'll be using the `{{ header }}` context to set the `:tocdepth:` of all pages (sphinx does not have a setting to set all the `:tocdepth:` at once).

In #24086 we added to most `.rst` files the header, that it's also used to import pandas and numpy, and set some settings. Adding it to the pages that don't have it yet.