Mention URLs in our error messages

We hear questions from our users about some of our error messages. @will noted that we could mention URLs (where appropriate) in our error messages to provide more info.

For example, we could use one of the attached FAQ errors as an example:

test=# SELECT COUNT(t.id) FROM "event" "t" LEFT JOIN "sensor" ON sensor.id = t.sensor_id;

ERROR: cannot perform distributed planning on this query

DETAIL: Shards of relations in outer join queries must have 1-to-1 shard partitioning. More info https://cit.us/102

-> we can always re-direct the content to be the right one, without necessarily a pull request later on.

I'm thinking another side benefit of this approach could be that we could infer which error messages frequently come up and confuse our users.

We could add this URL initially for #750, #949, #693, and #310.