[MINDEXER-232] There should be constants for the different index creator types and these should be documented somewhere

**[Martin Todorov](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=carlspring)** opened **[MINDEXER-232](https://issues.apache.org/jira/browse/MINDEXER-232?redirect=false)** and commented

# Description

There don't appear to be any constants for the possible index creator types. They also don't appear to be documented. This makes it harder to understand what index creator types exist and how they can be used.

# Task List

* (off) Define constants for the different index creator types.
* (off) Refactor the code so that it uses these constants.
* (off) Add documentation that outlines what these index creators types are used for.
* (off) Clarify what the default indexers are.
* (off) Clarify what indexers are used to create the indexes for Maven Central.

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No further details from [MINDEXER-232](https://issues.apache.org/jira/browse/MINDEXER-232?redirect=false)

The issue names no files or tests. Start by locating the existing index creator type definitions and their call sites, then identify the default and Maven Central indexers. Done means named constants replace the current type references and documentation explains each type and those defaults.

Written by the indexing model from the issue text.