By default, when separate HTML files are made, the SGML processor will assign arbitrary names to the resulting files. This can be confusing to readers who may bookmark a page only to have it change. Whatever your reasoning, here's how to make separate files named the way you want:
In your first <article> tag (which should be the only one) include an id parameter and call it index. This will make your tag look like this:
On the first <chapter> tag, do not modify it, as it's usually an introduction and you want that on the first page. For each other <section> tag, include the id parameter and name it. A name should include only alphanumeric characters, and it should be short enough to understand what it is.