Update Index and IndexInterface docstrings with RST formatting and code examples (#552)

## Summary

This PR updates all docstrings for methods in the `Index` class and the
underlying `IndexInterface` class to use proper RST syntax with
comprehensive code-block examples.

## Changes

- **Updated all Index class method docstrings** with:
  - Proper RST formatting using Args/Returns/Examples sections
  - Comprehensive code-block examples showing different usage patterns
- Proper whitespace formatting (empty lines after code blocks for Sphinx
rendering)

- **Updated IndexInterface class docstrings** to:
  - Match the Index class style and formatting
  - Use consistent RST formatting throughout
  - Include code-block examples for all methods
  - Remove old `:param:` and `:type:` syntax in favor of Args sections

## Methods Updated

All methods in both classes now have complete, up-to-date docstrings:
- `upsert`, `upsert_from_dataframe`, `upsert_records`
- `search`, `search_records`
- `delete`, `fetch`, `fetch_by_metadata`
- `query`, `query_namespaces`
- `update`, `describe_index_stats`
- `list`, `list_paginated`
- `create_namespace`, `describe_namespace`, `delete_namespace`
- `list_namespaces`, `list_namespaces_paginated`
- Import-related methods (`start_import`, `list_imports`, etc.)

## Impact

- Improved documentation quality and consistency
- Better IDE support with comprehensive examples
- Proper Sphinx rendering with correct whitespace formatting
- All docstrings now follow RST best practices

Author: jhamon