Add index.html to root of parser API docs page

[jrdouceur]

There is no default document on https://digitaltwinconsortium.github.io/DTDLParser/, which is the root of the parser API docs site https://digitaltwinconsortium.github.io/DTDLParser/api/DTDLParser.html. Navigating to this root yields a 404 error, which is potentially confusing. There should be a default document that redirects to the actual docs top-level page.