"Getting Started" guide for Luke [LUCENE-9459]

While Luke has been very popular, widely accepted tool to Lucene users (including Solr and Elasticsearch users) for 10+ years, it lacks good documentation and/or user guide. Although Luke is a GUI tool that describes itself, it's not good for new users.

The lack of documentation is partly due to my laziness though, there is some inherent difficulty of explaining such low-level tool; if you don't know Lucene you don't understand Luke's capability or usefulness, if you once understand Lucene at some level, it's obvious to you and no explanation is needed.  :)

Nonetheless, it would be great if we have "Getting Started" documentation for Luke on our web site for new users/devs.

We may be able to have a Markdown file with some screenshots and usage descriptions, then convert it to HTML by Gradle task,  so that we can publish it with whole API documentation.

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Migrated from [LUCENE-9459](https://issues.apache.org/jira/browse/LUCENE-9459) by Tomoko Uchida (@mocobeta), updated Sep 02 2020

Start by reviewing Luke and the existing web/API documentation publication flow, then identify where a Markdown guide and its Gradle conversion task belong. Cover installation or launch, core usage, screenshots, and the Lucene knowledge needed to understand Luke; done means the guide is published as HTML for new users.

Written by the indexing model from the issue text.