then in your anchor <a> element add a target attribute with a value equal to the iframe name’s value (in this casetarget="iframeName" ) … here is the final example for your first html file :
<iframe src="your default src" name="iframeName" height="200px" width="100%"></iframe>
<!-- here is your link that will be appear in the iframe -->
<a href="documentation.html#intro" target="iframeName"> open the link in the iframe </a>