feat: restructure user guide following Diataxis information architecture

Reorganise all documentation into Diataxis-aligned folders (getting-started,
tutorial, explanation, how-to, how-to/nfv, reference, developer-guide, others).
Each folder has its own index.rst for the Sphinx toctree hierarchy. Large source
files (05-osm-usage, 04-vim-setup, 06-advanced-cluster-management) are split into
topic-focused how-to guides. Developer guide content is sourced from the
feat/gitlab-workflow branch of osm-developer-guide. All internal cross-links and
image paths updated to reflect new folder depths.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>