[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.

Start with doc/getting_started/index.rst and the relevant API reference pages to identify links and terminology. Create doc/getting_started/glossary.rst covering the listed core concepts, compression parameters, container choices, and index types, then link it from the getting-started index; the glossary is done when every requested term has a concise definition and API reference link.

Written by the indexing model from the issue text.