Add docs for common SQL error codes 08003 and 08006

Richard Loveland (rmloveland) commented:

According to an internal convo the most common SQL errors seen by clients are:

+ [40001](https://www.cockroachlabs.com/docs/stable/common-errors.html#restart-transaction)
+ [40003](https://www.cockroachlabs.com/docs/stable/common-errors.html#result-is-ambiguous)
+ 08003
+ 08006

Luckily, the first two already have docs (see links above).

Therefore we need to update the [Common Errors](https://www.cockroachlabs.com/docs/stable/common-errors.html) page with descriptions of the latter two.

This is a stopgap until we eventually (?) create some infrastructure to auto-generate all of our error code docs (which according to said internal convo is what is needed for the linked issue below).

See also:

- cockroachdb/docs#806

gz#9901

gz#10264

Jira Issue: DOC-1288