RFR: 8314147: Updated the PhongMaterial documentation

[Nir Lisker]

Overhaul to the `PhongMaterial` documentation (and a bit to its superclass `Material`). Except for the introduction, I divided the documentation into 3 sections: qualitative explanation, mathematical model (I wouldn't think it necessary, but the current doc explains it), and examples.

The reason for the verbosity of the doc is that I envisioned 2 target audiences for this class. One is a Java developer who wants to understand the terminology and workings of computer graphics or of the artists who are already familiar with this domain. (How many Java developers know what diffuse, specular and normal maps are?) The other is an artist who is already familiar with the domain, but wants to see how this class compares with other renderers. For this reason, I looked at the terminology used by engines like Blender, Maya, UE4 and Unity and tried to mention the comparisons (like bump vs. height vs. normal maps, or specular vs. roughness/smoothness).

The examples I chose and some of the schematics are not the best, looking at it retroactively, but I want to give enough time for reviewers and get this into 22.

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Commit messages:
 - Whitespace
 - Initial commit

Changes: https://git.openjdk.org/jfx/pull/1378/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1378&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314147
  Stats: 637 lines in 41 files changed: 525 ins; 17 del; 95 mod
  Patch: https://git.openjdk.org/jfx/pull/1378.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1378/head:pull/1378

PR: https://git.openjdk.org/jfx/pull/1378