Add sidebar layouts

[MuhammadAashirAslam]

This PR migrates the sidebar navigation files to Hugo partial layouts. It handles the structure, check for active items, and recursive rendering of the side menu to match the new site structure.
Short description of the files:

• layouts/partials/sidebar.html: Selects the page sidebar data and starts rendering the navigation tree.
• layouts/partials/sidebar_is_web.html: Checks whether a sidebar item is enabled for web output.
• layouts/partials/sidebar_item_active.html: Recursively determines whether a sidebar branch contains the current page.
• layouts/partials/sidebar_page_matches_url.html: Matches a page against its permalink, URL, and aliases.
• layouts/partials/sidebar_recursive.html: Recursively renders sidebar leaves and Bootstrap collapse branches.
• layouts/partials/sidebar_resolve_url.html: Preserves external URLs and resolves internal URLs with relURL.
• layouts/partials/sidebar_url_clean.html: Normalizes URLs before sidebar comparisons.

[MakisH]

With a quick sanity check, this looks fine, and on the testing website the sidebars seem to work fine (assuming that the build is a bit outdated - the CalculiX adapter page "Building on SuperMUC" should not exist anymore since #962).

Does this already address #586 completely?