Clickable breadcrumbs

[Blargian]

Closes #2636. Introduces changes to clean up the navigation of docs together with .

NB - we will need to try merge this PR and the other at the same time

Changes

In many places we show an overview page as a sub-child of the category, such as here:

In my opinion this clutters the sidebar, especially when the overview page has the same name as the category. This PR makes a small change to the sidebarItemsGenerator such that any file ending index.md will not show as a sub-child of the category. The category itself is then given the link to the overview or index page and becomes clickable:

Further more, breadcrumbs often did not link to anywhere such as here, because 'introduction' was a category and not a specific page:

Index files are added (in a separate PR) in some places and categories in the sidebar linked such that breadcrumbs are clickable:

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/en/integrations/index.mdx

[Blargian]

DocsCheck fails:

Error:  Loading of version failed for version current
Error:  Unable to build website for locale en.
Error:  Error: Invalid sidebar file at "sidebars.js".
These sidebar document ids do not exist:
- en/engines/index
- en/sql-reference/functions/functions-index

This is because the former is added in #71846 over at main repo and the latter because en/sql-reference/functions/functions-index was previously en/sql-reference/functions/index. Rename happens in that PR as well.

May also need to add a redirect for that.