DOC-721 Add template for autogenerated index pages from filtered docs

[JakeSCahill]

This PR introduces a new get-index-data.js helper and corresponding index-list.hbs UI template to support autogenerating index pages with links that aren't required to be in the same nav tree section (as we already have in the index.hbs` template). These additions aim to enhance the organization, navigability, and user experience of the Redpanda docs site by enabling dynamic, categorized lists of documentation pages based on configurable criteria.

These index pages:

• Enable users to browse through categorized lists of documentation pages, enhancing their ability to find relevant information quickly.

• Ensure that indexes remain up-to-date as new pages are added or existing ones are modified.

• Provide a standardized layout and styling for all index pages, ensuring a cohesive look and feel across the docs site.

Here's an example of how this template will be used:

= Docker Compose Labs
:page-index-data: docker-labs-index
:page-role: index-list

• page-index-data refers to the name of the attribute that contains the data required to render the list on the page (an extension will handle this: DOC-721 Add extension to fetch data for index pages docs-extensions-and-macros#87).
• page-role tells the UI to use the index-list template to render this page, effectively rendering the list of data defined in page-index-data.

Here's an example of the rendered output:

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