Doc: Intro Update

[ax3l]

Updated the intro paragraphs with ideas we discussed in the weekly meeting.

[jlvay]

Looks good! A couple of comments/suggestions:

• remove "is" in "ImpactX is runs"
• perhaps add "(with time as the independent variable)" after "general particle-in-cell code"?

[ax3l]

Thanks, updated :)

[cemitch99]

This looks good to me. I suggest that there is a follow-up paragraph reproducing some of the text from the IMPACT-Z description (most importantly, examples of applications), an stating explicitly that tracking methods are symplectic. I can contribute this if we agree on where it should be located.

[ax3l]

Thanks, @cemitch99! Oh yes, I forgot that we wanted to reuse text from the readme in https://github.com/impact-lbl/IMPACT-Z.

Feel free to contribute here or we merge and do a follow-up PR to add more. I think in the As a beam dynamics code, ImpactX ... could be a good paragraph to extend with applications and symplectic properties.