Final changes for release notes 6.11

[GuiMacielPereira]

Added release notes from changes that were merged during release period.

Description of work

Final touches for release notes for 6.11

Summary of work

Added missing release notes, edited index page, fixed typos and grammatical errors and added a new gif.

Further detail of work

To test:

Build docs-html and read over release notes to look for typos or bad formatting.

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[GuiMacielPereira]

I am still waiting on Richard to make the note about the new algorithm FindMultipleUMatrices clearer