auto populate the content of _index.md pages

[phila82r]

e.g. when you go to https://docs.dfuse.io/ethereum/public-apis/getting-started/ you're basically looking at https://docs.dfuse.io/ethereum/public-apis/getting-started.md

The content of that page was manually created Text + links to sections) but it could be auto generated so that when we add a new page or sub-section, a link will appear on _index.md page.

[abourget]

Should use the menu variables to generate it with a template. Or remove them and redirect to the first sub-element