[DOC] Glossary page for core concepts

New contributors and users frequently confuse chunks, blocks, frames, and superchunks — terms that appear throughout the API but are never defined in one place. The getting-started section has no glossary.

Suggested work: Create doc/getting_started/glossary.rst (and link it from doc/getting_started/index.rst) defining at minimum:

• SChunk, frame, chunk, block, subblock
• Compression parameters (cparams): codec, clevel, filters
• NDArray vs CTable vs SChunk — when to use each
• Index types: SUMMARY, BUCKET, PARTIAL, FULL, OPSI — one-sentence each

Each term should link to the relevant API reference page.