docs: remove projects as an allowed resource type from project level policy declarations documentation

[hubwoop]

Tried to use this policy:

p, proj:my-proj:my-role, projects, get, my-proj, allow

on a project, but got the following error:

Unable to update project: invalid policy rule 'p, proj:my-proj:my-role, projects, get, my-proj, allow': project resource must be: 'applications', 'applicationsets', 'repositories', 'exec', 'logs' or 'clusters', not 'projects'

which seems to be in line with:

argo-cd/pkg/apis/application/v1alpha1/types.go

Line 2518 in ec9b43f

return status.Errorf(codes.InvalidArgument, "invalid policy rule '%s': project resource must be: 'applications', 'applicationsets', 'repositories', 'exec', 'logs' or 'clusters', not '%s'", policy, resource)

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[blakepettersson]

Thanks!