feat: add uptime-since-acquisition metric

[jphuynh]

This is a bit of a cheeky change coming from a conversation with people saying that they feel like the uptime is getting worse since the acquisition of GitHub by Microsoft.

Adds a "Other stats" panel at the bottom showing the uptime percentage since October 26, 2018 excluding scheduled maintenance windows.

[mrshu]

Still open and still merges cleanly. Two notes now that today's changes have landed:

• The all-time chart from site: Add all-time graph #24 shipped, which gives a lifetime figure (95.30%) from a different angle — 90-day rolling uptime plotted since Jun 2022. Your "uptime since acquisition" metric is a different and more pointed cut, so I don't think it's redundant, but it's worth checking the two don't contradict each other visually when both are on the page.
• Please double-check your metric against Headline "GitHub Platform Uptime" metric is easy to misinterpret #21, which argues the existing headline is easy to misread as "how often GitHub works" when it actually measures all-systems-green time. Any new aggregate on the page inherits that ambiguity, so a label that makes the definition explicit would help.

Happy to merge once you've had a look.