Merge Twentysix Index cog into Downloader

### Type of feature request

Cog, Other

### Description of the feature you're suggesting

It was previously discussed (and even voted on iirc) but there's nothing tracking it at present

The reason for this is to further improve the user experience while interacting with 3rd Party cogs and Downloader by making all of the repos in the indexer available from a fresh install of Red.

Source: https://github.com/Twentysix26/x26-Cogs
Index: https://index.discord.red/

### Anything else?

It would be a huge usability boon for new users (and current users, especially if Option 1/2 is picked in #5940

Review the Downloader behavior alongside the Twentysix26/x26-Cogs source and the index at https://index.discord.red/. Check the related discussion in #5940 for the referenced options. Done means a fresh Red installation can access all repositories listed in the index through Downloader.

Written by the indexing model from the issue text.