exporters: make "creating a custom exporter" more prominent

[odeke-em]

We currently have a section showing folks how to create custom exporters but from offline conversations, seems like this section is hard to find. Let's make it jump off the page when someone visits the /exporters The menu currently looks like

but let's make it standout more under "Exporters". It currently looks like this https://opencensus.io/exporters/

/cc @hvent90 @shahprit

[shahprit]

I would say explicitly make "getting started - creating a new exporter"