Add/expand introduction

[jorainer]

Feedback from @annatagliaferri

have a general introduction to explain:

• LC-MS
• MS1
• MS2
• whole concept why we have this workflow
• emphasize need for adaptability

[philouail]

I will have my next push without these fixes as this is a bigger change and we need to organize.

@Danmarques29 you are our "expert" in the more technical part of LC-MS/MS. DO you think you could try to improve our intro especially for LC-MS relevancy and MS1 vsMS2 explanation ?
We will have also some visual support. it doesn't need to be extra detailed, but informative enough for new users :)

I will work on the concept/adaptability to reflect our core aims

[philouail]

Update: Dan added some info on MS1 and MS2 in the intro :)