feat: create docs for Assets API endpoint (#614)

Author: LuizAsFight

packages/graphql/ASSETS.md

@@ -0,0 +1,32 @@
+# Fuel Explorer Indexer - Assets API
+### Instructions for consuming Assets data from Indexer API
+
+API endpoint:
+`/assets/<assetId>`  - get information about any asset (including NFTs)
+
+URLs to consume:
+- Mainnet: https://explorer-indexer-mainnet.fuel.network
+- Testnet: https://explorer-indexer-testnet.fuel.network
+
+
+`curl` example of usage
+```
+curl 'https://explorer-indexer-mainnet.fuel.network/assets/0x44b09d7143aa7b1aac7844ee9cfc38893b12d20c3822ecb18dbf2c6846ea63f0'
+```
+
+
+the result contains a JSON with the asset information, here are the details about each prop:
+| Property | Description |
+|----------|-------------|
+| assetId | Actual ID of the asset |
+| contractId | Contract ID that minted the asset |
+| subId | SubId used to calculate the assetId |
+| name | Asset name |
+| symbol | Asset symbol |
+| decimals | Asset decimals configuration |
+| suspicious | true if symbol matches any verified asset symbols |
+| metadata | Key-value object with SRC7/SRC9 standard metadata |
+| isNFT | true if asset is NFT ( [supply=1, decimals=0](https://docs.fuel.network/docs/sway-standards/src-20-native-asset/#non-fungible-asset-restrictions) )|
+| verified | true if asset is in Fuel's verified assets list |
+| owner | Current assetId owner (NFTs only) |
+| uri | Parsed URL from “metadata.uri” or “metadata.URI”. Some [logic](https://github.com/FuelLabs/fuel-ex-demo/blob/ee73408a0ef0323eeb92414d5ed92d65e3f3f78f/packages/graphql/src/infra/gateway/AssetGateway.ts#L63-L95) is applied to try defining a reasonable uri for the API consumer |