Document custom query rules

[deepthi]

Custom rules can be provided to vttablet using the filecustomrules flag. We need to document this in a website guide that lists all supported functionality.

References: https://github.com/vitessio/vitess/blob/master/go/vt/vttablet/tabletserver/rules/rules_test.go https://github.com/vitessio/vitess/blob/master/go/vt/vttablet/customrule/filecustomrule/filecustomrule_test.go

[alexrs]

If possible, it would be good to document the difference between topocustomrule and filecustomrule.

[harshit-gangal]

@deepthi do you know anyone from community who are using this? They would better know how they are doing that and might be interested to document it?