RST: Simple Tables: multiple header rows not supported

[ZoomRmc]

========== =========
Header A1  Header A2
Header B1  Header B2
========== =========
body a1    body a1
body b1    body b2
========== =========

The header rows are parsed as body and the header separator becomes a row with HorizontalRules.

Relevant part of the spec:

A simple table begins with a top border of equals signs with one or more spaces at each column boundary (two or more spaces recommended). Regardless of spans, the top border must fully describe all table columns. There must be at least two columns in the table (to differentiate it from section headers). The top border may be followed by header rows, and the last of the optional header rows is underlined with '=', again with spaces at column boundaries. There may not be a blank line below the header row separator; it would be interpreted as the bottom border of the table.

Perhaps would be reasonable to implement jointly with #10127
Related: #6701

Pandoc 3.5