This seemed quite easy so I wrote my own plugin for the website physiotherapy exercises in a day or so.
You can use any server technology you like to provide the content, it does not need to be ASP NET.
You can create an office plugin using Visual Studio 2015 community edition, more details can be found here.
A tutorial for developing your adding can be found here.
Microsoft provide their own (html based) UI Toolkit, called fabric for developing office apps. It can be found here.
You can get a free developer account with a one year subscription to office 365 and then submit your app on the seller dashboard.
It took a few tries for me to get the app approved, it can take up to 5 days each time you submit. Some tips I learnt from my mistakes :
- Make sure your application version is exactly the same, including all the decimals (for example 220.127.116.11).
- Don't include any taskbar in your screenshots, it will result in your screenshots getting rejected
- Read the guidelines carefully