DOC: Improve CSV/TSV output documentation table, include `type` and `example`

[corneliusroemer]

@chaoran-chen raised that it'd be helpful to have type and example in the table documenting the nextclade.tsv/csv output file.

This is a good idea, it's not hard to add. The issue is more how to properly display it. Right now, on rtd, the width of the table is severely limited making even 2 columns difficult to show side-by-side.

Alternative display is some sort of flat format, or hosting the table somewhere else, like Github and linking to it from rtd.

[ivan-aksamentov]

On a related note, more dev-oriented, I played with csv validator schema a little, back in the day: https://github.com/nextstrain/nextclade/blob/master/packages/schemas/nextclade_tsv.csvs This allows checking that we emit what we really mean to emit. The tool is not great though and the schema format is super weird too.

[corneliusroemer]

Design improvements address the problem of the table being chopped off after 800px

See https://github.com/nextstrain/sphinx-theme/pull/18?notification_referrer_id=NT_kwDOAX_wQbMzMjgyNTE1NjUyOjI1MTYxNzkz

Now we can add extra columns for type and example @chaoran-chen