Document Enhanced Guides scripting API updates

[JessiMcM]

Summary

Documents the guide-related scripting API, covering both the original guide
methods (After Effects 16.1) and the new GuideOptions-based API added in
After Effects (Beta) 26.5.

• docs/item/item.md - Item.guides, Item.addGuide(),
  Item.getGuideAsObject(), Item.setGuide(): version-based property tables
  and both call signatures (integer form + GuideOptions form).
• docs/layer/layer.md - Parallel documentation for the layer-level guide
  methods (Layer.guides, Layer.addGuide(), Layer.getGuideAsObject(),
  Layer.setGuide()), which have existed since 16.1 but were previously
  undocumented.
• docs/other/guideoptions.md - New GuideOptions object page with the
  GuideOrientationType and GuidePositionType enumerations.
• docs/introduction/changelog.md - Adds the 26.5 entry and fills in the
  16.1 guide entries.
• mkdocs.yml - Adds the GuideOptions page to the nav.

Additional callouts

• APIs new in 26.5 Beta (GuideOptions, the enum types, getGuideAsObject(), and
  the GuideOptions overloads of addGuide()/setGuide()) carry a note that
  they are in Beta and subject to change.
• A "Differences between guide versions" callout explains that the integer
  values behind orientationType/positionType differ between versions, so
  code should compare against the named enumerated constants rather than raw
  integers in the beta.