Update About page: mission statement, sponsorship details, founding year

[jstac]

Summary

• Add a mission statement section explaining QuantEcon's role and open-source/open-science commitment
• Clarify QuantEcon's fiscal sponsorship arrangement with NumFOCUS and PSL Foundation (tax status, trademark holding, governance)
• Add founding year (2015) to the History section and minor copy tweaks elsewhere

Test plan

• Preview rendered about page to confirm markdown formatting/links are correct

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[jstac]

@jdebacker, this is an update to our mission statement. @thomassargent30 flagged the fact that we should alert you to these changes. Please let us know if you have any comments.

[mmcky]

thanks @jstac. I'll review and read this in detail this afternoon.

[jdebacker]

@mmcky and @jstac All of these changes look good from the perspective of PSL Foundation. Thanks for flagging me.

[jstac]

Thanks @jdebacker .

@mmcky Please go ahead and feel free to merge when ready.

[mmcky]

thanks @jstac + @jdebacker