feat(info): show driver docs URL in dbc info

[djfrancesco]

Closes #499

Summary

dbc docs already uses the docs_url from the registry index, but dbc info never showed it. This PR adds it to both the plaintext and the JSON output.

In plaintext, there's a new Docs: line after Description:. It only shows up when the driver has a docs URL:

$ dbc info snowflake
Driver: snowflake
Version: 1.14.0
Title: ASF Snowflake Driver
License: Apache-2.0
Description: An ADBC driver for Snowflake developed under the Apache Software Foundation
Docs: https://adbc-drivers.org/drivers/snowflake/
Available Packages:
   - linux_amd64
   ...

With --json, the driver.info payload gets a docs_url field:

{"schema_version":1,"kind":"driver.info","payload":{"driver":"snowflake", ..., "description":"...","docs_url":"https://adbc-drivers.org/drivers/snowflake/","packages":[...]}}

A few choices I made

I went with Docs: as the label since it's short and matches dbc docs. Drivers without a docs URL get no line at all, so their output doesn't change.

In the JSON, docs_url is always there (empty string when unset). I left out omitempty on purpose, so it behaves like title, license and description right next to it. The field is new, so schema_version stays at 1.

Easy to change any of these if you'd prefer something else.

The tests reuse the existing test-driver-docs-url fixture, and also check that a driver without a docs URL gets no Docs: line and an empty docs_url.

Not in this PR

dbc search -v --json doesn't include docs_url either. I can do that in a follow-up if you want it.

Test plan

• go test ./... passes with Go 1.26.8 (env and user levels)
• New tests fail without the change and pass with it
• go vet ./... clean; changed files are gofmt-clean
• Manual: dbc info snowflake and dbc info snowflake --json against the default registry show the docs URL