Remove unused mkdocs-section-index package

A built-in feature of `mkdocs-material` is used instead.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

pyproject.toml

@@ -64,7 +64,6 @@ dev-mkdocs = [
   "mkdocs-literate-nav == 0.6.1",
   "mkdocs-macros-plugin == 1.0.4",
   "mkdocs-material == 9.3.1",
-  "mkdocs-section-index == 0.3.7",
   "mkdocstrings[python] == 0.23.0",
   "frequenz-repo-config[lib] == 0.6.1",
 ]