doc: requirements: update Sphinx and rtd theme

rtd_theme 1.2.x is required when using Sphinx >= 6.0, otherwise certain
features like search are broken. Note that jQuery support needs to be
enabled manually now using `sphinxcontrib.jquery` extension. Also update
Sphinx to latest 6.x release, 6.2 as it contains some fixes.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>