Write index release note page for v6.9

[robertapplin]

Description of work

This PR adds the highlights of v6.9 to the release note index page. It also updates the image - in this case it is a small collage of two of the new algorithms added to Mantid.

To test:

Check the index page for any mistakes. Make sure the image is rendered as expected.

Reviewer

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Code Review

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• Does the code conform to the coding standards?
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• Do changes function as described? Add comments below that describe the tests performed?
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[backmari]

Other than the one comment about the warning, the index page looks good to me! The links are working, except for two that I believe will be generated on the release date: the DOI for the release and the GitHub release page.

[sf1919]

DOI and Github release links will be sorted out at release as you correctly summise @backmari :)

I'm happy so I will get this merged in.