Migrate HTTP SD docs from docs repo (#8972)

See discussion in https://github.com/prometheus/docs/pull/1975

Signed-off-by: Julius Volz <julius.volz@gmail.com>

docs/disabled_features.md

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 title: Disabled Features
-sort_rank: 10
+sort_rank: 11
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 # Disabled Features

docs/http_sd.md

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+---
+title: HTTP SD
+sort_rank: 7
+---
+
+# Writing HTTP Service Discovery
+
+Prometheus provides a generic [HTTP Service Discovery](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#http_sd_config),
+that enables it to discover targets over an HTTP endpoint.
+
+The HTTP Service Discovery is complimentary to the supported service
+discovery mechanisms, and is an alternative to [File-based Service Discovery](https://prometheus.io/docs/guides/file-sd/#use-file-based-service-discovery-to-discover-scrape-targets).
+
+## Comparison between File-Based SD and HTTP SD
+
+Here is a table comparing our two generic Service Discovery implementations.
+
+| Item | File SD | HTTP SD |
+| ---- | ------- | ------- |
+| Event Based | Yes, via inotify | No |
+| Update frequency | Instant, thanks to inotify | Following refresh_interval |
+| Format | Yaml or JSON | JSON |
+| Transport | Local file | HTTP/HTTPS |
+| Security | File-Based security | TLS, Basic auth, Authorization header, OAuth2 |
+
+## Requirements of HTTP SD endpoints
+
+If you implement an HTTP SD endpoint, here is a few requirements you should be
+aware of.
+
+The response is consumed as is, unmodified. On each refresh interval (default: 1
+minute), Prometheus will perform a GET request to the HTTP SD endpoint. The GET
+request contains a `X-Prometheus-Refresh-Interval-Seconds` HTTP header with the
+refresh interval.
+
+The SD endpoint must answer with an HTTP 200 response, with the HTTP Header
+`Content-Type: application/json`. The answer must be UTF-8 formatted.
+If no targets should be transmitted, HTTP 200 must also be emitted, with
+an empty list `[]`. Target lists are unordered.
+
+Prometheus caches target lists. If an error occurs while fetching an updated
+targets list, Prometheus keeps using the current targets list. The targets list
+is not saved across restart.
+
+The whole list of targets must be returned on every scrape. There is no support
+for incremental updates. A Prometheus instance does not send its hostname and it
+is not possible for a SD endpoint to know if the SD requests is the first one
+after a restart or not.
+
+The URL to the HTTP SD is not considered secret. The authentication, and any API
+keys should be passed with the appropriate authentication mechanisms. Prometheus
+supports TLS authentication, basic authentication, OAuth2, and authorization
+headers.
+
+## HTTP_SD format
+
+```json
+[
+  {
+    "targets": [ "<host>", ... ],
+    "labels": {
+      "<labelname>": "<labelvalue>", ...
+    }
+  },
+  ...
+]
+```
+
+
+Examples:
+
+```json
+[
+    {
+        "targets": ["10.0.10.2:9100", "10.0.10.3:9100", "10.0.10.4:9100", "10.0.10.5:9100"],
+        "labels": {
+            "__meta_datacenter": "london",
+            "__meta_prometheus_job": "node"
+        }
+    },
+    {
+        "targets": ["10.0.40.2:9100", "10.0.40.3:9100"],
+        "labels": {
+            "__meta_datacenter": "london",
+            "__meta_prometheus_job": "alertmanager"
+        }
+    },
+    {
+        "targets": ["10.0.40.2:9093", "10.0.40.3:9093"],
+        "labels": {
+            "__meta_datacenter": "newyork",
+            "__meta_prometheus_job": "alertmanager"
+        }
+    }
+]
+```

docs/management_api.md

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 title: Management API
-sort_rank: 7
+sort_rank: 8
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 # Management API

docs/migration.md

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 title: Migration
-sort_rank: 8
+sort_rank: 9
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 # Prometheus 2.0 migration guide

docs/stability.md

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 title: API Stability
-sort_rank: 9
+sort_rank: 10
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 # API Stability Guarantees