Remove pinned version of grafana/docs-base

What?

Ensures the latest shortcodes and other Hugo website templates are available in local docs builds.

[x] I have used a meaningful title for the PR.
[x] I have described the changes I've made in the "What?" section above.
[x] I have performed a self-review of my changes.
[x] I have run the npm start command locally and verified that the changes look good.