Documentation of the end point for fetching a data-var

[radicleart]

I believe you can get the value of a data var in a clarity contract through API but it hasn't been documented;

E.g. this work but isn't in the official api docs.

/v2/data_var/${contractAddress}/${contractName}/${dataVarName}

[ryanwaits]

ty for flagging @radicleart we'll add this in our api reference with our next docs release

[ryanwaits]

closing as this is part of a larger api update to reference and link to rpc endpoints #761