[DOCS-13274] Add Bits Assistant page to NDM docs

[git-thuerk-done]

What does this PR do? What is the motivation?

Fixes DOCS-13274

Adds a new Bits Assistant page to the Network Device Monitoring docs, documenting how to use Bits Assistant and the Datadog MCP Server to query and troubleshoot network devices with natural language.

Changes:

• New page at network_monitoring/devices/bits_assistant covering Bits Assistant usage (device lookup, interface troubleshooting, traffic analysis, configuration auditing) and MCP Server tools
• Added Bits Assistant to NDM side nav (after Configuration Management)
• Added NDM skills section to the main Bits Assistant page
• Added link on the NDM index page (also removed a duplicate Integrations entry)

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/alicia.thuerk/ndm-bits-assistant/network_monitoring/devices/ai_tools

Modified Files

• https://docs-staging.datadoghq.com/alicia.thuerk/ndm-bits-assistant/bits_ai/bits_assistant
• https://docs-staging.datadoghq.com/alicia.thuerk/ndm-bits-assistant/network_monitoring/devices/