Migrate docs from Sphinx to MkDocs (#18145)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

requirements/docs.txt

@@ -1,19 +1,8 @@
-sphinx==7.4.7
-sphinx-argparse==0.5.2
-sphinx-book-theme==1.1.4
-sphinx-copybutton==0.5.2
-sphinx-design==0.6.1
-sphinx-togglebutton==0.3.2
-myst-parser==3.0.1  # `myst-parser==4.0.1` breaks inline code in titles
-msgspec
-snowballstemmer<3  # https://github.com/snowballstem/snowball/issues/229
-commonmark # Required by sphinx-argparse when using :markdownhelp:
-
-# Custom autodoc2 is necessary for faster docstring processing
-# see: https://github.com/sphinx-extensions2/sphinx-autodoc2/issues/33#issuecomment-2856386035
-git+https://github.com/hmellor/sphinx-autodoc2.git # sphinx-autodoc2==0.5.0
-
-# packages to install to build the documentation
-cachetools
--f https://download.pytorch.org/whl/cpu
-torch
+mkdocs
+mkdocs-api-autonav
+mkdocs-material
+mkdocstrings-python
+mkdocs-gen-files
+mkdocs-awesome-nav
+python-markdown-math
+ruff