Reorder OSS "Get Started with Flux" page to be more clear

This page would be more clear if we reordered the sections to put the "Before you start" section first and then introduced the tools for working with Flux.

Also update intro statement:
These guides walk through important concepts related to Flux and querying time series data from InfluxDB using Flux.

To:
Before you start, walk through important Flux concepts, and then review tools to query time series data from InfluxDB using Flux.

New user feedback!
Page: https://docs.influxdata.com/influxdb/v2.3/query-data/get-started/
Feedback: i dont know what you mean sry :(

Open the OSS “Get Started with Flux” page at docs.influxdata.com/influxdb/v2.3/query-data/get-started/ and locate its source in the docs repository. Put “Before you start” before the tool introductions and update the introductory statement to the wording in the issue. Done means the page order and text match the requested changes.

Written by the indexing model from the issue text.