docs: Switch to our own fork of sphinxcontrib-openapi
docs: Switch to our own fork of sphinxcontrib-openapi

Sphinxcontrib-openapi relies on m2r. But m2r is unmaintained [0]. It
relies on mistune, but is not compatible with mistune versions >= 2.0
[1]. This makes it impossible to update mistune, even though there is a
CVE in the old versions of the package [2].

Switch to sphinx-mdinclude instead.

There is also m2r2, but it is not a suitable alternative because they
simply force the use of an older mistune version [3].

[0] https://github.com/sphinx-contrib/openapi/issues/123
[1] https://github.com/miyakogi/m2r/issues/66
[2] https://github.com/advisories/GHSA-fw3v-x4f2-v673
[3] https://github.com/CrossNox/m2r2/pull/43

Signed-off-by: Quentin Monnet <d6b8e48afb2534b213e391cab43016505747a234@isovalent.com>

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