Improve error message and documentation on the needed link between router and service

[mloiseleur]

What does this PR do?

It improve the situation described in #10258 by:

1. Provides only complete examples in the doc
2. Improve wording in the doc on this matter
3. Improve error message (with the router name when it's possible).

Motivation

Improve UserXP without breaking change, since many users faced it in the last four years. Detailed in #10258.

More

• Added/updated documentation

Additional Notes

This PR aims to improve User XP with the current behavior of Docker provider.
Introducing a new Convention over configuration (aka magic) way for configuring Traefik Proxy is not the purpose of this PR.
=> This PR does not supersede #10252.
Thanks to @joshka and @bluepuma77 who can be credited for my motivation on this PR.

[ldez]

In 90% of the cases, an explicit service label is useless.

It's better to add one example of an explicit definition instead of modifying all the current examples.

[mloiseleur]

In 90% of the cases, an explicit service label is useless.

It's better to add one example of an explicit definition instead of modifying all the current examples.

@ldez @bluepuma77 @mmatur With ceed9c5, I tried to improve the wording and keep current "magic" example. I also added a complete example in service definition section.

[kevinpollet]

Hello @mloiseleur,

Could you please rebase this pull request on branch v2.11?

[mloiseleur]

Hello @kevinpollet ,

PR has been rebased.

[rtribotte]

LGTM