(website) Update shape docs to include details of where clause optimisation

[robacourt]

Following on from #2182 on @thruflo 's suggestion, this PR add's where clause optimisation documentation to the main docs and links to it from various places. This includes what types of where clauses are optimised.

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[thruflo]

@robacourt if you don't mind I'm just going to do a quick copy pass on this. Just to ask:

For optimised where clauses, we can process 4000 - 6000 row changes per second, regardless of how many shapes there are. For non-optimised where clauses, we can process 140k row changes per second, per shape. For more details see the benchmarks.

Is this correct? Because it's a bit confusing to me. The copy refers to millions of where clauses at once but then we can only handle 4000 - 6000 row changes per second. Whereas non-optimised we can do 140k?

Ideally I'd like to express the total or per shape throughput so that it's higher for optimised where clauses.