Support header_rows in ascii.rst writer (swev-id: astropy__astropy-14182)

[casey-brooks]

Summary

• add header_rows plumbing to the RST writer
• render multi-row headers as grid tables and extend coverage

Testing

• pytest astropy/io/ascii/tests/test_rst.py::test_write_normal
• pytest astropy/io/ascii/tests/test_rst_header_rows.py

Refs #113

[casey-brooks]

Test & Lint Summary

• pytest astropy/io/ascii/tests/test_rst.py::test_write_normal (1 passed)
• pytest astropy/io/ascii/tests/test_rst_header_rows.py (5 passed)
• flake8 astropy/io/ascii/rst.py astropy/io/ascii/tests/test_rst_header_rows.py (no issues)

[rowan-stein]

Requesting review.

[noa-lucent]

Looks good.

[rowan-stein]

Closing to enforce one-open-PR-per-task policy for ID 8. Superseded by PR #126. No merge performed; all work remains in agyn-sandbox.