[docs] Add community libraries page to Related Libraries

Nobody has claimed this yet.

We should have a page linking to useful 3rd party libraries to help developers find them.

We might want to have some criteria for inclusion, so that we link to well-maintained libraries. What well-maintained means is a big debate though, but we want to send users to libraries that will work for them.

Potential categories:
- Tools, like bundler or IDE plugins
- Integrations with other libraries (like Redux, Tanstack, etc)
- Lit-specific utilities
- _Extendable_ components and design systems. We don't want to make a component catalog here, because web components are interoperable it doesn't generally matter if they're built with Lit or not, but people may be looking for a design system implemented with Lit so that they can utilize base classes, helpers, etc., to extend it with their own components. Lit developers may also look for extendable components that have APIs that take Lit render-props, etc. like a Lit-specific table or tree.
- Starter kits

Some potential libraries and tools:

- https://open-wc.org/docs/development/lit-helpers/
- https://www.npmjs.com/package/postcss-lit
- https://github.com/adobe/lit-mobx
- https://www.npmjs.com/package/@tanstack/lit-form
- https://github.com/JulianCataldo/vite-plugin-standard-css-modules
- https://shoelace.style/
- https://opensource.adobe.com/spectrum-web-components/
- https://www.opencells.dev/
- https://github.com/Conectate/ct-elements
- https://www.npmjs.com/package/rollup-plugin-postcss-lit
- https://web-components.carbondesignsystem.com/
- https://github.com/SAP/ui5-webcomponents/ (though this one is a bit weird - I think they compile handlebars into lit-html)
- https://github.com/momentum-design/momentum-ui/tree/master/web-components
- https://www.npmjs.com/package/lit-redux-router
- https://github.com/43081j/relit
- https://tanstack.com/table/latest/docs/framework/lit/lit-table
- https://github.com/bennypowers/lit-css
- https://www.npmjs.com/package/@vaadin/form
- https://www.npmjs.com/package/eslint-plugin-lit
- https://www.npmjs.com/package/eslint-plugin-lit-a11y
- https://www.npmjs.com/package/lit-scss-loader
- https://github.com/apollo-elements/apollo-elements

There's a lot more at https://github.com/web-padawan/awesome-lit too

Start from the site's Related Libraries entry point and review the candidate libraries and tools listed in this issue, along with the linked awesome-lit collection. Before adding content, clarify the inclusion criteria with maintainers; done means a categorized community-libraries page is published with useful, maintained links.

Written by the indexing model from the issue text.