Tweak the display of insertion text

[damithc]

Current:

Suggested: Something like this?

What do you think? We can omit the inserted by: CustardUI part if mentioning the plugin name in the middle of the main text feels intrusive.

I also realised the current one doesn't allow center-aligning of text.

[gerteck]

Yes, agreed that the current styling of the <cv-insertions> was a interim design, it would be better to make it simple, and allow some attribute options of updating hte border, and text alignment etc.

Will open a PR to update this along with updating the docs!