fix(html): prevent hierarchy reset in rich table cells (#2716)

* fix(html): restore parents after rich cell walking

Signed-off-by: Matvei Smirnov <vdalekesmirnov@gmail.com>

* fix(html): add table cell context manager, update tests

Signed-off-by: Matvei Smirnov <vdalekesmirnov@gmail.com>

* fix(html): table with heading test data

Signed-off-by: Matvei Smirnov <vdalekesmirnov@gmail.com>

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Signed-off-by: Matvei Smirnov <vdalekesmirnov@gmail.com>