docs(device-discovery): vrf discovery via the discover_vrfs option

[leoparente]

Summary

Documents the new discover_vrfs device-discovery policy option shipped in netboxlabs/orb-discovery#452:

• Options table: discover_vrfs (bool, default false) — discovers VRFs via the driver's get_network_instances() and attaches them to the IP addresses and prefixes of each VRF's member interfaces.
• New [VRFs] section in the README: precedence (discovered VRF wins over defaults.*.vrf / vrf_ipv4 / vrf_ipv6 for member interfaces), instance filtering rules (default instance, L2 types, platform-internal __-prefixed names), and route-distinguisher handling including the Diode rd-matching caveat.
• What NAPALM Collects Automatically: the IP Address and Prefix VRF rows now reflect auto-collection when the option is on.
• Supported platforms page: new VRFs driver matrix — ios, eos, junos, nxos, nxos_ssh in this first batch (upstream NAPALM getter); other drivers land in follow-up batches.

Draft until the orb-discovery PR merges and ships in a device-discovery release.

🤖 Generated with Claude Code

[leoparente]

Updated the VRFs driver matrix for the first custom-driver batch (netboxlabs/orb-discovery#453): iosxr, iosxr_netconf, nokia_sros, nokia_srl move to Supported with per-driver data-source notes.

[leoparente]

Updated the VRFs driver matrix for the second custom-driver batch (netboxlabs/orb-discovery#454): huawei_vrp, hp_comware, cumulus_linux, dell_sonic move to Supported with per-driver data-source notes.

[leoparente]

Updated the VRFs driver matrix for the third custom-driver batch (netboxlabs/orb-discovery#455): aruba_aoscx, nokia_sros_ssh, cisco_viptela_ssh, paloalto_panos, paloalto_panos_ssh move to Supported. 18 of the planned drivers are now covered.

[leoparente]

Updated the VRFs driver matrix for the fourth and final custom-driver batch (netboxlabs/orb-discovery#456): extreme_slx, dell_ftos, brocade_netiron, extreme_vsp move to Supported (the latter three flagged as built from vendor-documented formats pending live-device validation). The matrix close-out also lands in this push: 22 drivers supported, 14 documented as permanently N/A (no VRF concept), and the remaining VRF-capable platforms listed as demand-driven with their mapping rules pre-decided. This completes the planned VRF discovery coverage.