[EPIC] Improvement to docs

### Describe the proposal

The current docs for Gravitino has a lot of repeated and redundant materials. Navigating through the docs is not a pleasant experience, especially for new users. We need to restructure the docs for clarity, conciseness and readability.

### Task list

The list of tasks will be dynamically changed on demand.

- [x] Improve the glossary documentation
- [x] Restructure the getting started tutorial
- [x] Refactor the build guide.
- [x] Restructure the whole documentation hierarchy
- [x] Revise the wide markdown tables to HTML tables (narrow tables can be skipped)
- [x] Ensure all the code snippets in the examples are clean and runnable