docs: quote the network label error in the guide, not just describe it

[jravani]

Follow-up to #44, one paragraph in the getting-started guide, no behaviour change.

#44 said the error text was recorded in the compose files, the guide and ADR-005. It was in the first and last only. The guide described the failure without quoting it, so the string com.docker.compose.network appeared nowhere on the published site:

published getting-started page, before:  0 occurrences
after:                                   1

That misses the reader #44 was written for. A compose file is greppable only with the repository already checked out, and the arc42 ADR ships as a PDF. The guide is the published page someone lands on after pasting an error message into a search, which was the whole stated purpose.

The message is now quoted, and the two cases are distinguished by the value it reports rather than by prose, since they need different fixes:

• empty label: the network was created outside Compose, by docker network create. Remove it.
• any other value: a compose file declared the network under a different key. Compose compares the key, not the name:.

Verified the string survives into the rendered HTML rather than only into the Markdown source, which is the mistake that produced this in the first place. mkdocs build --strict exit 0.