Split Beginner Course modules 1-6 into navigable subpages

The left nav renders each module as a <details> group whose submenu lists
the module's child pages. Modules 1-6 kept all their content inline in
_index.md, so those groups expanded to nothing and the modules could not
be opened from the nav. Module 0 already had subpages and worked.

- Split each module's numbered sections into one subpage per section,
  matching module 0 and the Essentials/Multi-Vector courses. Each module's
  _index.md keeps its video, overview, and Today's Path, now linked to the
  subpages. Trailing "What's Next" / "Where to Go" is appended to the
  module's last subpage.
- Rewrote 22 "Section N" cross-references as links to the new pages, plus
  four "above"/"next section" references that became cross-page.
- Certification moved from certification/_index.md to certification.md
  with an explicit url, matching multi-vector-search. The nav renders
  non-section pages with .Params.url, so as a childless section it was
  unreachable too. URL is unchanged.
- Added the Beginner Course to the Learn nav above Essentials.
- The advance button read "Complete Module 1: Let's Understand Search";
  it now reads "Complete Module 1".

No prose was changed beyond the reference rewrites above. Module 1's
Today's Path summary bullets were replaced by the linked section list
used in every other module.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

qdrant-landing/content/learn/ref-courses-beginners.md

@@ -0,0 +1,11 @@
+---
+#Delimiter files are used to separate the list of documentation pages into sections.
+type: reference
+reference: /course/beginners
+weight: 205
+sitemapExclude: True
+build:
+  publishResources: false
+  render: never
+partition: learn
+---