Modify DeveloperGuide.adoc to use smart links
Modify DeveloperGuide.adoc to use smart links

When linking to other asciidoctor documents, we use the extension .adoc.

The extensions remain unchanged even when exporting to HTML.

Let's fix this by using smart links, such that asciidoctor will change
to the correct extension .html when exporting to HTML.

damithc/addressbook-level4,damithc/addressbook-level4,se-edu/addressbook-level3,CS2103R-Eugene-Peh/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4