Call it SpriteStudio

[Naruto]

Every mention of the authoring tool now reads SpriteStudio — no OPTPiX
prefix, no 7 suffix. 22 occurrences across 15 files.

Two reasons, both about what the name has to survive:

• OPTPiX is the brand, not the product. It carries no information for
  anyone integrating a player, and nothing on these pages ever explains it.
• 7 dates the page. The tool keeps going past 7, and nothing here is
  specific to 7 — leaving the number in means editing every page again when 8
  lands.

What changed

READMEs and docs/, the mkdocs nav label and site_description, and the doc_classes XML — the text Godot shows in its own help panel.

What did not

• spritestudio7 inside webtech.co.jp URLs — 6 of them, untouched.

Checked before the sweep: no line here contrasts 7 with SpriteStudio 6, so
dropping the number costs no meaning.

The vendored SpriteStudio-SDK submodule is untouched — it carries the same
change in its own PR and arrives here on the next pointer bump.