[DOCS] Add Quick Start page

[jrodewig]

Creates a high-level, tutorial-based introduction to Elasticsearch and its basic features.
Resolves #40861.

Welcome any and all feedback.

Firebase preview

https://es-demo-docs.firebaseapp.com/quick-start.html

Notes

• I intentionally avoided more complex or architecture-related concepts, like nodes and clusters.
• I left out Java requirements. It looks like we'll be bundling JDK with our distro in 7.0+.
• My examples used data from the Kibana accounts sample set. Happy to update this if we prefer to use the more common twitter examples.
• I'm not particularly happy with the instances of 'a document with an ID of X' throughout. I'd appreciate any suggestions for alternate wording.

[elasticmachine]

Pinging @elastic/es-docs

[elasticmachine]

Pinging @elastic/es-search

[lcawl]

This section reminds me a lot of https://www.elastic.co/guide/en/elastic-stack-get-started/master/get-started-elastic-stack.html#install-elasticsearch

If you agree, perhaps we can combine the content and just re-use it in both places (e.g. via a tagged region). I'm happy to help with that.

[jrodewig]

That sounds great! I'll go ahead and add a tag to this section. I'll look into combining the two docs as part of another issue/PR.

[lcawl]

I generally try to avoid future tense, but it's not a hard rule.

Suggested change

	<<docs-index_, index API>>. {es} will automatically generate an ID for the
	<<docs-index_, index API>>. {es} automatically generates an ID for the

[lcawl]

I added a few minor comments but it builds successfully and generally LGTM. If you're still encountering gradle errors and need help debugging, let me know!

[jrodewig]

The @esdocs team met and discussed this PR. Here's a summary:

• We currently risk proliferating too many "Getting Started" and "Quick Start" guides across the Elastic Stack. This is particularly true for installation instructions.
• There are ongoing efforts to consolidate this type of content in the Getting Started with Elastic Stack guide.
• @kat257 and others will lead efforts to inventory our current "start" documentation.
• To prevent further proliferation, we'll close this PR.