Clickable breadcrumbs

[Blargian]

Closes #2636. Introduces changes to clean up the navigation of docs together with 71846 over on main repo.

PR will fail due to two missing files which were added in the PR on the main repo

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:

Also addresses 2888, adding landing pages where they are missing.

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.