Getting started guide for InfluxDB 3 Core and Enterprise lacks sha256 of the install script

While convenient, `curl | sh` is not best practice (even when served over HTTPS; consider the `codecov` incident). At a minimum, please add the sha256sum for the install script in these sections. You could alternatively update the command to verify the sha256sum as part of the invocation (similar to how telegraf (et al) verify the gpg key in https://www.influxdata.com/downloads/).

Note: the install script correctly verifies the sha256 of the file it downloads. This is about ensuring that the sha256 of the install script itself can be verified.

##### Relevant URLs
- https://docs.influxdata.com/influxdb3/core/get-started/#run-the-install-script
- https://docs.influxdata.com/influxdb3/enterprise/get-started/#run-the-install-script

Open the linked InfluxDB 3 Core and Enterprise getting-started pages and inspect their “Run the install script” sections. Add the install script’s sha256sum or an invocation that verifies it before execution, then confirm both sections provide a usable verification step.

Written by the indexing model from the issue text.