Homepage: add data attributes to CTAs

[elzannewentzel]

There are CTA's on the homepage that needs the data-button-style, data-button-location and data-button-type attributes added.

On the homepage, for the use cases section:
Add the attributes to the 5 “Learn more about… ” CTA’s

On the homepage, under the 1.2M engineers heading:
Add the attributes to the “Learn how our customers use Sourcegraph” CTA

[bretthayes]

@elzannewentzel I updated our Tracking doc to reflect these updates.