Add doc for integrating with FE isolated positions via API. (#227)

pages/api_integration-guides/how_to_isolated.md

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+# How to integrate APIs with FE isolated positions
+
+## Overview
+This document covers how to use the API to query data on / trade with isolated positions that are managed with the dYdX V4 front-end.
+
+## Isolated Positions on dYdX V4 front-end
+Isolated positions on the dYdX V4 front-end are perpetual positions held in subaccounts with a subaccount number greater than 127, up to the limit of 128,000. Each isolated position is held in a separate subaccount.
+
+### Mapping of isolated positions to subaccounts
+The dYdX V4 front-end implementation separates subaccounts (0 - 128,000) into 2 separate types:
+
+**Parent subaccounts**
+
+Subaccounts 0 to 127 are parent subaccounts. Parent subaccounts can have multiple positions opened and all positions are cross-margined.
+
+**Child subaccounts**
+
+Subaccounts 128 to 128,000 are child subaccounts. Child subaccounts will only ever have up to 1 position open. Each open isolated position on the front-end is held by a separate child subaccount.
+Once an isolated position is closed in the front-end, the subaccount associated with isolated position can be re-used for the next isolated position.
+Child subaccounts are mapped to parent subaccounts using the formula:
+```
+parent_subaccount_number = child_subaccount_number % 128
+```
+
+> Note that currently only parent subaccount 0 is exposed via the front-end and so isolated positions will be held in subaccounts number 128, 256, ...
+
+> Note that the above "types" of subaccounts are not enforced at a protocol level, and only on the front-end. Any subaccount can hold any number of positions in cross-marginable markets which all will cross-margined at the protocol level.
+
+### Getting data for parent subaccount
+API endpoints exist to get data for a parent subaccount and all it's child subaccounts on the Indexer.
+
+> Currently all data for an account viewable on the front-end can be fetched by using the parent subaccount APIs to fetch data for parent subaccount number 0.
+
+See the <a href="/developers/indexer/indexer_api">Indexer API</a> page for more details of the parent subaccount APIs.