Track the docs Hugo module and drop the blanket dependencies label

[Ilyes512]

Two small changes to .github/dependabot.yml.

Watch the docs Hugo module

docs/ is a Go module of its own — the Hextra theme is consumed as a Hugo module from docs/go.mod, which the root module never sees. It had no Dependabot entry, so theme updates were invisible. This adds a second gomod ecosystem for /docs, with the same weekly schedule, build commit prefix and grouped minor/patch updates as the root one.

Drop the dependencies label

Every ecosystem carried dependencies alongside its specific label (github-actions, go, docker). The generic one added nothing on top of the specific one, so it is gone; each ecosystem now applies exactly one label.

Notes

• Labels are applied by Dependabot, not by labelsync — this is repository plumbing only, no code change, so no tests or docs updates apply.
• The dependencies label still exists on existing open Dependabot PRs; it just will not be applied to new ones.