docs: update README with Analytics info

[machikoyasuda]

In tandem with this ticket in MobiMart, cal-itp/mobility-marketplace#610, wanted to use this PR to document:

• calitp.org is NOT sending data to Amplitude.
• calitp.org is being tracked by Google Analytics, version 4, with enhanced measurement turned on for outbound links, file downloads and more.
• adding a note in the README about this so new users know where to look for reporting.

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