[next-gen docs] Update next-gen capitalization of terms and also trim down on the getting started guide

[hello-ashleyintech]

Summary

This PR accomplishes the following:

• Updates next-gen terms to lowercase per this discussion
• Update next-gen Getting Started guide so it's trimmed down more, aligns more with Bolt Python's guide, and also is easier to follow

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #1709 (42bbf1e) into main (527276f) will not change coverage.
The diff coverage is n/a.

❗ Current head 42bbf1e differs from pull request most recent head 3872753. Consider uploading reports for the commit 3872753 to get more accurate results

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[srajiang]

Left a couple of suggestions for other things that maybe shouldn't be capitalized when used at the start of a sentence - Datastores, Manifest, namely.

And then also some nits on wording!

[srajiang]

LGTM!