docs: add some for utils

Author: janek26

www/docs/API/changelog.md

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 # CHANGELOG

www/docs/API/utils.md

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+sidebar_position: 6
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+
+# Utils
+
+Util functions are provided so you can use low level functions in your application.
+
+## `address`
+
+the address helpers can be imported using:
+
+```js
+import { address } from 'starknet.js';
+```
+
+### `getChecksumAddress(address: BigNumberish): string`
+
+This function accepts an address as a `BigNumberish` and returns the checksummed address as a string.
+An example:
+
+```js
+import { address } from 'starknet.js';
+
+const addressToCheck = '0x2fd23d9182193775423497fc0c472e156c57c69e4089a1967fb288a2d84e914';
+
+const checksummedAddress = address.getChecksumAddress(addressToCheck);
+
+console.log(checksummedAddress); // 0x02FD23D9182193775423497Fc0c472E156C57C69E4089a1967fb288a2D84e914
+```
+
+### `validateChecksumAddress(address: string): boolean`
+
+This function validates the checksum address. It returns true if the address is valid, false otherwise.