docs(document-translation): document Cat 1 file formats (IDML, XML, JSON, DITA, MIF)

[yoshiamakino]

Summary

• Add IDML, XML, JSON, DITA, and MIF to all supported-format listings (OpenAPI YAML/JSON, api-reference/document.mdx, docs/getting-started/about.mdx, Java cookbook).
• Update XLIFF version wording everywhere to "versions 1.2, 2.0, and 2.1".
• Update docs/resources/usage-limits.mdx: new rows for IDML (30 MB Pro), MIF (30 MB Pro), XML (10 MB), JSON (1 MB), DITA (5 MB); bump Word / PowerPoint / PDF from 30 MB → 100 MB on API Pro; rewrite XLIFF footnote.
• Enrich docs/best-practices/document-translations.mdx:
  • Note the correct per-format size limits.
  • Add One source/target language pair per upload — behaviour is single-language-per-upload for most formats, with an XLIFF exception (per-segment) and a billing note that all content counts toward billed characters.
  • Add Same-language source and target are rejected (HTTP 400, incl. regional variants).
  • Add Format-specific gotchas covering XML (only text between tags; CDATA translated; ITS external rules 500), XLIFF (target overwrite; per-<file> lang ignored), DITA (.dita only; codeblock newline collapse), JSON (1 MB limit; embedded HTML/Markdown), IDML (font substitution; overset), and MIF (must be FrameMaker; UTF-8; MIF ≥ 8.00).

Test plan

• Mintlify preview renders docs/best-practices/document-translations.mdx cleanly (headings, bullet spacing, code spans).
• docs/resources/usage-limits.mdx table renders with correct new rows and 100 MB values for Word / PowerPoint / PDF (API Pro).
• /api-reference/document shows the expanded format list in both the top listing and the file parameter description.
• Auto-generated /api-reference/document/upload-and-translate-a-document picks up the OpenAPI updates.
• docs/getting-started/about.mdx one-liner still reads cleanly.
• Java cookbook snippet still parses.