docs: expand README with usage examples

Replaces the placeholder with full usage docs modelled on chime's
README but tighter — gloss is a smaller package, the doc shouldn't
out-bulk it.

Sections: nav links, status, why-not-quick-sdcv, features, installation
(package-vc-install / use-package :vc / straight / manual), quick start
(lookup and add flows), the C-h g keybinding table plus the
gloss-add-mode binding table, the four defcustoms with rationale,
extending the source registry for v2+, org-drill integration,
troubleshooting (libxml absent, network failures, file corruption,
cache out of sync, side-window dismiss), development (Cask + ert-runner
+ Make targets), license.

The keybinding table is the source of truth — every entry matches what
`gloss-prefix-map' actually binds. The defcustoms section names the
four actual customs only (gloss-file, gloss-fetch-sources,
gloss-fetch-timeout, gloss-debug); no aspirational knobs.

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