doc: add QoS flows handling and PDU Session Modify documentation

Also, move PDU Session Release diagram to new PDU Session Management section in rrc-dev.md.

Changes:

PDU Session Modify:
- Update PDU session modification sequence diagram
- Add DRB-To-Remove/To-Modify/To-Setup list handling
- Document E1AP Bearer Context Modification flow
- Add RRC reconfiguration trigger after E1AP response

QoS Flows Handling:

- Complete overview with 3GPP standards references (TS 23.501, 37.324, 38.463, 29.281, 38.331)
- Comprehensive Mermaid sequence diagram covering control and data plane

Technical details:
- Multiple QoS flows per DRB supported
- QFI to DRB mapping at RRC and SDAP layer
- F1-U tunnels: 1 per DRB, no QFI marking
- N3 tunnels: 1 per PDU session, with QFI marking
Signed-off-by: Guido Casati <guido.casati@openairinterface.org>

doc/E1AP/e1ap_procedures.md

@@ -65,36 +65,3 @@ sequenceDiagram
   u->>c: BEARER CONTEXT MODIFICATION RESPONSE
   Note over c: e1apCUCP_handle_BEARER_CONTEXT_MODIFICATION_RESPONSE
 ```
-
-## PDU Session Release
-
-```mermaid
-sequenceDiagram
-    participant AMF
-    participant CUCP
-    participant CUUP
-    participant DU
-    participant UE
-
-    AMF->>CUCP: NG PDU SESSION RESOURCE RELEASE COMMAND
-    Note over CUCP: ngap_gNB_handle_pdusession_release_command
-    CUCP->>CUCP: rrc_gNB_process_NGAP_PDUSESSION_RELEASE_COMMAND
-    Note over CUCP: set status PDU_SESSION_STATUS_TORELEASE
-    CUCP->>CUUP: E1 Bearer Context Modification Request
-    Note over CUUP: release_gtpu_tunnel (GTP tunnel, PDCP, SDAP)
-    CUUP->>CUCP: E1 Bearer Context Modification Response
-    Note over CUCP: rrc_send_f1_ue_context_modification_request
-    CUCP->>DU: F1 UE Context Modification Request
-    Note over DU: handle_ue_context_drbs_release (release in MAC/RLC)
-    DU->>CUCP: F1 UE Context Modification Response
-    Note over CUCP: rrc_CU_process_ue_context_modification_response
-    Note over CUCP: replace existing CellGroupConfig
-    Note over CUCP: rrc_gNB_generate_dedicatedRRCReconfiguration
-    CUCP->>UE: RRCReconfiguration (DRB release list, NAS PDU)
-    UE->>CUCP: RRCReconfigurationComplete
-    Note over CUCP: handle_rrcReconfigurationComplete
-    Note over CUCP: rrc_gNB_send_NGAP_PDUSESSION_RELEASE_RESPONSE
-    CUCP->>AMF: NG PDU SESSION RESOURCE RELEASE RESPONSE
-    Note over CUCP: rm_drbs_by_pdusession (from stored RRC list)
-    Note over CUCP: rm_pduSession (from stored RRC list)
-```