docs(reference): Document v6 note templates and Marp-style image resizing

[craftzdog]

Summary

Documents two features shipping in Inkdrop desktop v6 (v6.0.0-rc.0):

• Note templates — a new dedicated reference page, plus a mention of the template picker in the Write notes page.
• Image resizing — rewritten to cover the native Marp-style size specifiers and the new image resize toolbar.

Changes

Note templates — new page /reference/note-templates

• The Templates section on the sidebar — browsing built-in + custom templates (Custom section first, then built-ins by category).
• Create a note from a template — the Create a new note picker (search, live preview, Recently used), plus a Blank-note tip.
• Built-in templates — category overview (Brainstorming, Debugging, Learning, Planning, Productivity).
• Create a custom template — a note in the built-in Templates notebook; customizing a built-in one by editing it directly (Inkdrop forks it into a Custom copy on save, leaving the original intact).
• Tags & status — recommended via the editor, with the _template frontmatter as an alternative.
• _template frontmatter — title, description, tags, notebook, status; names/IDs accepted for tags and notebook; dynamic date / uuid placeholders; guidance blocks (> # !Instructions) that are stripped from generated notes and are handy for directing an AI agent.
• Added Note templates to the Note organization nav group, and surfaced the picker in Write notes.

Image resizing — /reference/resize-images

• Replaced the obsolete custom-CSS #small workaround with the v6 native feature:
  • the Image Size toolbar (Auto / Small / Half / Full),
  • Marp-style width: / height: (and w: / h:) specifiers, supported units, and bare-number-as-pixels.

Screenshots

Placeholder images are committed for the sidebar, template picker, resize toolbar, and resize example — swap in final captures as needed.

Test plan

• npm run build passes locally (routes.json regenerated; /reference/note-templates and /reference/write-notes prerender)
• Nav shows Note templates under Note organization
• Page titles/descriptions come from frontmatter (not the default fallback)
• Internal links resolve (write-notes ↔ note-templates, note-statuses, mcp-server)
• Screenshots display correctly once finalized