Add a documentation style guide for consistent, on-brand docs

[chet]

Docs authored by hand drift in voice, formatting, and NVIDIA branding because we have no written convention for prose the way STYLE_GUIDE.md covers Rust. This adds a documentation style guide that any contributor or agent can load before writing docs, so the output stays consistent and on-brand no matter who writes it.

What this involves

• Add DOCS_STYLE_GUIDE.md at the repo root: prose and formatting conventions for user-facing docs (branding, PACE voice, word choice, numbers, punctuation, grammar, formatting, lists and tables, links, accessibility, readability), an over-application-safeguards section, and a quick checklist. Aligned with NVIDIA's documentation style.
• Keep it at the root, deliberately outside docs/, so it stays an authoring reference and is not published as a page on the Fern docs site.
• Wire discovery so it actually gets used: reference it from AGENTS.md (agents writing docs), STYLE_GUIDE.md (the sibling for Rust code style), and CONTRIBUTING.md (human contributors).