Add highlights to the 6.10 release notes index page

[backmari]

Description of work

Add highlights to the 6.10 release notes index page. Minor edits.

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[backmari]

If there are other suggestions for image for the index page, please let me know!

[sf1919]

I'm wondering if we need to make it clear what IDAaaS is. Can we use ISIS Data Analysis as a Service Platform (IDAaaS) the first time it is mentioned and then use the acronymn IDAaaS afterwards?

[backmari]

@jhaigh0 @sf1919 Thank you for the comments and having a look at the PR!