Generate the content of the root ecosystem page

[thibaultamartin]

https://github.com/matrix-org/matrix.org/blob/main/content/ecosystem/_index.md is plain text, we can generate the content (in a shortcode?) from the actual data instead

[MTRNord]

Followup on #2190 and #2187

[MTRNord]

Note on impl: Its probably easier to generate this via a template as you there get access to the subcategory data directly.