Add Measurement and Scoring details to about page

[camillobruni]

Given that this comes up ever so often in discussion with internal and external folks it might be worthwhile documenting in a bit more detail which parts are measured and how we calculate the final score.

• Add example timeline to explain which parts contribute to the score
• Add basic formulas on how the test steps are combined into a final score

[camillobruni]

After fixing the paragraphs this is ready for reviews.

[bgrins]

Instead of the about page, could we put this type of information in a markdown file in the repo? That way it wouldn't require publishes to browserbench.org (presumably requiring a version bump) to update the content.

[camillobruni]

I can put it into a separate html file, markdown is likely the wrong file format for math formulas and tables, no?

[bgrins]

I think you can embed the HTML into markdown to preserve the ability to host the preview directly out of the github repo but no strong opinion either way.

[camillobruni]

I've not pulled out the timeline table and the scoring math into separate SVGs (this seems to be the least brittle way to get externally formatted content into markdown).

WDYT?

[bgrins]

Thanks. We can iterate on it in the future as necessary