Adopt the get_MNV docs style and a dark-mode-aware logo

[Paururo]

Brings the documentation site into the same visual family as get_MNV, and rolls the new SNPick logo across the repo with a dark-mode-aware treatment.

Style

• Palette repainted to get_MNV's: white ground in light mode, black in dark, a single teal accent used only for links and controls, and the IBM Plex type family.
• The look from get_MNV's extra.css ported into docs/stylesheets/extra.css: hairlines instead of boxes, headings that open on a rule, toned-down callouts, and hairline tables.
• The nav is grouped into sections (Get started, Reference, Project) shown open in the sidebar; tabs are off, matching get_MNV on a site this small.

Logo, adapted to dark mode

• The new SNPick.svg becomes the logo everywhere.
• Its wordmark is set in the "Sixtyfour Convergence" pixel font, which a visitor's browser will not have, so the site ships baked light and dark PNGs: the pixel shapes render as raster for everyone, not as a plain fallback face.
• A light and a dark variant are swapped by colour scheme in three places: the masthead (a logo.html partial override), the landing hero (#only-light / #only-dark), and the README (<picture> with prefers-color-scheme).

Notes

• Verified with a local mkdocs build and by eye in both light and dark mode.
• logo/SNPick.pdf still holds the previous drawing; it is unreferenced in the repo and was left for a separate vector re-export.
• README carries some pre-existing em-dashes unrelated to this change.