docs: add Elixir page (exqlite run-time loading + static compilation for iOS)

[AlanMcCann]

Adds a site/using/elixir.md page following the existing per-language format, plus the sidebar entry.

Covers:

• Run-time loading through exqlite (enable_load_extension + load_extension) and per-connection loading via Ecto's load_extensions config.
• Static compilation into the exqlite NIF via SQLITE_EXTRA_INIT + sqlite3_auto_extension — the only option on iOS, where dynamic code loading is forbidden. References the EXQLITE_EXTRA_SRC hook proposed in Add EXQLITE_EXTRA_SRC: statically compile SQLite extensions into the NIF elixir-sqlite/exqlite#355 and notes the two-line Makefile patch as the interim.
• Packing float lists into the little-endian float32 BLOB format (the Elixir equivalent of the .pack("f*") snippet on the Ruby page).

Everything in the page is verified by execution in a production Elixir app: sqlite-vec v0.1.9 statically compiled into exqlite's bundled SQLite (3.51.3 and 3.53.4), known-answer cosine checks (vec_distance_cosine(vec_f32('[1,0]'), vec_f32('[0,1]')) = 1.0), including an iOS device-SDK static archive with execution verified in the simulator.