Add "HTTP Service Discovery" section to integrations docs, similar to File SD section

[tjhop]

Currently, the integration docs have a section showcasing some File SD integrations from the community.

I propose we add a similar section for HTTP Service discovery, highlighting some HTTP SD implementations from the community. Here's an initial list, for sake of discussion:

• fastly-exporter: https://github.com/fastly/fastly-exporter?tab=readme-ov-file#service-discovery
• ns1_exporter: https://github.com/tjhop/ns1_exporter?tab=readme-ov-file#http-service-discovery
• netbox, via plugin: https://github.com/FlxPeters/netbox-plugin-prometheus-sd
• netbox, via export templates: https://www.observability.blog/post/netbox_http_service_discovery/
• TDB