Add concept documents under docs/concepts for cross-cutting service behaviors

Add onboarding-oriented concept documents for service behaviors that belong to no single package, under docs/concepts/{concept}/KNOWLEDGE.md with type: concept.

Initial topics (5): session lifecycle end to end; deployment and replica; vfolder and the storage path; the RBAC scope and role model; scheduling (sokovan).

Design already settled in BA-7303: one directory per concept keeps the one-KNOWLEDGE-per-package rule; the existing search.py and check.py tooling applies unchanged because discovery is filename-based; each document opens with why the concept exists and links to the owning packages' KNOWLEDGE.md documents in the body.

JIRA Issue: BA-7305

Create one docs/concepts/{concept}/KNOWLEDGE.md document for each of the five listed behaviors, using type: concept. Start by reviewing the existing search.py and check.py tooling and the design settled in BA-7303; each document is done when it explains why the concept exists and links to the owning packages' KNOWLEDGE.md files.

Written by the indexing model from the issue text.