Replace the section-index plugin with navigation.indexes

The `section-index` plugin has a bug[^1] that causes the navigation bar
to display a wrong text for the **API Reference** link. There is a fix
for it, but `mkdocs-material` already provides the feature natively[^2],
so we can just use it instead, as it is much less likely to have issues.

[^1]: oprypin/mkdocs-section-index#18
[^2]: https://squidfunk.github.io/mkdocs-material/setup/setting-up-navigation/#section-index-pages

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

RELEASE_NOTES.md

@@ -14,4 +14,4 @@
 
 ## Bug Fixes
 
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+- Fix the **API Reference** link in the documentation website navigation bar.

mkdocs.yml

@@ -24,6 +24,7 @@ theme:
   features:
     - content.code.annotate
     - content.code.copy
+    - navigation.indexes
     - navigation.instant
     - navigation.tabs
     - navigation.top
@@ -115,7 +116,6 @@ plugins:
             - https://typing-extensions.readthedocs.io/en/stable/objects.inv
             - https://watchfiles.helpmanual.io/objects.inv
   - search
-  - section-index
 
 # Preview controls
 watch: