New Thumbnail Contrib How-To Guide, and phrasing/format revisions

[OctopusButtons]

Added detailed how-to guide for Contributing Thumbnails. Guide has been iterated, tested, and successfully-recruity-of-at-least-one-new-thumbnailer in a separate forum. Revised, proofread. (I'm the original author.)

Also revised other sections of Playlists and Thumbnails page:

• Custom Thumbnails. Heavy revision/formatting, with an eye toward expanding some lines for full-lucid-understanding but while also condensing and removing some redundancy
• Added Anchor links for "see below" "see above" references, in cases where there's a header to link. (I skipped cases where referenced bit isn't worth a header expanding the TOC on final site.)
• "Game Name" terminology clarification bubble and edits to game name passages
• More specific walkthrough explanation of Flexible Name Match Algorithm
• Edited phrasing and formatting for Custom Logos/Icons in a way that should help understanding and maybe SEO. (The synonym "icons", since the original idea may be for logos but people wondering about customizing this part of interface will probably search for "icons".)

Since the page is combined "Playlists and Thumbnails", and is large, we're probably due to separate into 2 pages. I see the site map doc structure here and will work on the division later as well as a new Database docs page.

Related Items to Come: - Connections between the new Thumbnail Contribution How-To and the thumbnail repository Readme

• I will PR to add a set of screenshotting guidelines to the thumbnail repository ReadMe rather than to the User Guide Docs. Reasoning: people who don't need a "Users"-centric doc guide for the github process will still most likely look at the repository ReadMe when contributing.
• Will also PR text/link for the ReadMe to direct people to the Contribution How-To in user docs, after (if) this PR gets approved and I verify the final page is live and AOK for outside linking.
• The doc guide in this PR already references reading all rules in the thumbnail repository readme.