Document the marker feature following the gns3-documentation standard:
overview, Mermaid architecture and business-process diagrams, API endpoint
tables, request/response and field reference, error responses, and notes
covering the key design points — immutable name (rename = delete + create),
global-prefix reservation, read-only inherited markers, render hints
(color / highlight_duration), supported node types, and persistence.
Indexed under Features in docs/README.md.
