Add Simon Hettrick's Keynote Speech

[mrmundt]

Description

This PR adds the Keynotes page, Simon Hettrick's details, and structure for Sandra's (as soon as it's available).

NOTE: The img is already there for Sandra; everything is currently commented out so it won't render.

Checklist:

• (Committee Chairs Only) I have posted the link for the PR in the usrse slack (#usrse-2024-conference-planning-committee) to ask for content reviewers
• I have previewed changes locally
• I have updated the README.md if necessary

cc ADD GITHUB HANDLES HERE

[mrmundt]

Preview: https://output.circle-artifacts.com/output/job/e52e899e-662c-4b42-965b-f9d57dcff982/artifacts/0/usrse.github.io/usrse24/program/keynotes/index.html

[mrmundt]

@jbteves-sandia - you aren't showing up in the reviewers list so TAG!