docs: Add API docs for v2

Add a markdown page for API docs

API.md

@@ -0,0 +1,121 @@
+# API
+
+API for accessing information about Sponge downloads
+
+Root URL: https://dl-api.spongepowered.org/
+
+* [/v2/groups](#v2groups)
+* [/v2/groups/\<groupCoordinates>/artifacts](#v2groupsgroupcoordinatesartifacts)
+* [/v2/groups/\<groupCoordinate>/artifacts/\<artifactId>](#v2groupsgroupcoordinateartifactsartifactid)
+* [/v2/groups/\<groupCoordinate>/artifacts/\<artifactId>/versions](#v2groupsgroupcoordinateartifactsartifactidversions)
+
+### /v2/groups
+Lists available groups.
+
+Example: GET https://dl-api.spongepowered.org/v2/groups
+```json
+{
+    "type": "Groups",
+    "groups": [
+        {
+            "groupCoordinates": "org.spongepowered",
+            "name": "SpongePowered",
+            "website": "https://spongepowered.org/"
+        }
+    ]
+}
+```
+
+### /v2/groups/\<groupCoordinates>/artifacts
+List available artifacts for a group.
+
+Example: GET https://dl-api.spongepowered.org/v2/groups/org.spongepowered/artifacts
+```json
+{
+    "type": "Artifacts",
+    "artifactIds": [
+        "spongevanilla",
+        "spongeforge"
+    ]
+}
+```
+
+### /v2/groups/\<groupCoordinate>/artifacts/\<artifactId>
+List metadata for an artifact, including available tags.
+
+Example:
+GET https://dl-api.spongepowered.org/v2/groups/org.spongepowered/artifacts/spongeforge
+```json
+{
+    "type": "latest",
+    "coordinates": {
+        "groupId": "org.spongepowered",
+        "artifactId": "spongeforge"
+    },
+    "displayName": "SpongeForge",
+    "website": null,
+    "gitRepository": "https://github.com/SpongePowered/SpongeForge",
+    "issues": null,
+    "tags": {
+        "api": [
+            "8.1",
+            "7.4",
+            "..."
+        ],
+        "forge": [
+            "2838",
+            "36.2.5",
+            "..."
+        ],
+        "minecraft": [
+            "1.16.5",
+            "..."
+        ]
+    }
+}
+```
+
+### /v2/groups/\<groupCoordinate>/artifacts/\<artifactId>/versions
+List available versions for an artifact. This list can be filtered based on URL parameters.
+
+URL Parameters:
+- `tags`: This is a dynamic list of values an artifact may be tagged with. This follows a `key:value` mapping, with
+  multiple tags being requested seperated by a comma. Example tags include minecraft and api. Example: `minecraft:1.16.5`
+  or `minecraft:1.16.5,api:8.1`. By default, no tags are filtered.
+- `recommended`: Whether to only include recommended builds. By default, this is `false`.
+
+Example: GET https://dl-api.spongepowered.org/v2/groups/org.spongepowered/artifacts/spongeforge/versions?recommended=true&tags=minecraft:1.12.2,api:7.4
+```json
+{
+    "artifacts": {
+        "1.12.2-2838-7.4.7": {
+            "tagValues": {
+                "minecraft": "1.12.2",
+                "forge": "2838",
+                "api": "7.4"
+            },
+            "recommended": true
+        },
+        "1.12.2-2838-7.4.6": {
+            "tagValues": {
+                "minecraft": "1.12.2",
+                "forge": "2838",
+                "api": "7.4"
+            },
+            "recommended": true
+        },
+        "1.12.2-2838-7.4.5": {
+            "tagValues": {
+                "minecraft": "1.12.2",
+                "forge": "2838",
+                "api": "7.4"
+            },
+            "recommended": true
+        },
+        "...": {}
+    },
+    "offset": 0,
+    "limit": 25,
+    "size": 8
+}
+```