docs: rewrite README to match wrapper documentation standard

[sayalibhavsar]

The problem

The current README is a 5-line stub with no documentation of command-line options, workflow, dependencies, examples, or troubleshooting.

Expected Behaviour

README should provide comprehensive, source-code-accurate documentation following the team's standard format (autohpl-wrapper style), covering:

• All command-line options (hammerdb-specific and general_setup)
• Step-by-step workflow for both hammerdb and run_hammerdb scripts
• Per-database dependencies (MariaDB, PostgreSQL, MSSQL)
• Benchmark explanation, output files, results schema
• Usage examples, storage layout, return codes, troubleshooting

Actual Behaviour

README contains only a title, one-line description, and upstream link. No operational documentation exists.

Relates to JIRA: RPOPC-937