Support live preview for new entries to sites.json

[damithc]

Adding the following to sites.json required a restart of the server to see the page http://127.0.0.1:8080/site-demo-cs2103/book/common/References.html.

    {
      "src": "book/common/References.md",
      "title": "CS2103/T Textbook - References"
    }

[damithc]

@alyip98 can you check if this issue is still applicable?

[ang-zeyu]

Very much still applicable