docs(DEVELOPER_GUIDE): Make getting started comprehensive and reorganize

Enhance getting started guide for new contributors
- Add key technology breakdown with links to code guidelines
- Add robust prerequisite guide
- Clarify forking workflow
- Explain bootstrapping
- Distinguish between recommended and optional/alternative steps
- Add OS-specific guidance
- Add tarball usage instructions

Make other developer guide improvements:
- Update TOC
- Remove callout linking to EUI docs
- Reorder code guidelines
- Standardize header capitalization and make usage guide headers imperative

fixes opensearch-project#3396

Signed-off-by: Josh Romero <rmerqg@amazon.com>