Mkdocs-section-index

* Prepare for readthedocs theme in MkDocs 1.6 release - https://github.com/oprypin/mkdocs-section-index/commit/1577f42c979a82fa0a083da56858912969361450

* Adapt template rewrite path to MkDocs-Material 9.4 - 20

* [Declare that 'nature' theme is supported](https://github.com/oprypin/mkdocs-section-index/commit/0ee7339c5196f64691b79ecb9abe8891254bd9f1) - #19
* [Fix setting the parent of a SectionPage](https://github.com/oprypin/mkdocs-section-index/commit/9bbe1bcf1f8d70043e9c43f9523ef611ee4642c1) - this was resulting in incorrect "breadcrumbs" display in "readthedocs" theme

* Adapt to mkdocs-material version 9.2 - 46dd8e72d5abcf2bb10b306556ddd31c5ec04172, 18

* Compatibility with mkdocs-material 9.0.7+: https://github.com/oprypin/mkdocs-section-index/commit/9e3ac74df822a066f6a2130d663bfb8286b399c9, #13

* mkdocs-material: fix displaying arrow for sections with 1 child page: https://github.com/oprypin/mkdocs-section-index/commit/4ad5a24a19ea420a8fd581641e41474f51c8cb19, #9