Pad out page footer

[kimandrews]

Description of proposed changes

Adds more info to the page footer, modeled after what is currently done for mpox

• Copies description.md from mpox
• Makes measles-specific changes to description.md and adds this info to the page footer

Addresses https://github.com/nextstrain/measles/issues/37

Related issue(s)

https://github.com/nextstrain/measles/issues/37

Checklist

• [ ] Checks pass

[kimandrews]

Screen shot of the page footer:

[trvrb]

This looks great to me @kimandrews. Can merge now and adjust "Reusing code and images" later depending on consensus.