Improve `convert` and `convertTo` pages on site

Documentation Improvements

Convert

1. Fix the operation grammar at the top of the page: replace the obsolete to { colExpression } form with the current .asColumn { colExpression } API.

2. Add a short notNull section with an example for converting nullable columns while preserving nulls.

3. Expand perRowCol from a grammar-only mention into a documented scenario: use it when conversion needs both the current row and the current column.

4. Add a compact “Specialized date/time conversions” section covering toLocalDate(pattern), toLocalDateTime(format), Java date/time overloads with DateTimeFormatter or locale, and Long/Int timestamp conversion with TimeZone.

5. Document the current Instant situation: toStdlibInstant, toDeprecatedInstant, and the planned migration of toInstant to kotlin.time.Instant.

6. Add examples for less visible supported targets: toUtcOffset, toYearMonth, toDateTimeComponents, toDuration, and toJavaDuration.

7. Expand toDataFrames(containsColumns = true) with a small example showing both row-oriented and column-oriented nested lists.

8. Add examples for URL -> IMG and URL -> IFRAME, including parameters such as width, height, and border.

9. Mention cast() briefly if it is intended as a user-facing API for adjusting the static column type before a conversion chain.

10. Clarify that ParserOptions are only used for String/Char parsing and ignored for non-string source columns.

ConvertTo

1. Make the distinction between DataFrame.convertTo<Schema>() and DataColumn.convertTo<ValueType>() even more prominent near the top, because the names are easy to confuse.

2. Add a short explanation of how convertTo<Schema>() uses the same automatic conversions as convert { ... }.to<T>(), linking back to the supported-types list on the convert page.

3. Add an example showing schema conversion with date/time parser options or a custom parser for date-like strings.

4. Explain precedence more explicitly: custom converters, parsers, and fillers override automatic conversions.

5. Add a small example for excessiveColumns = ExcessiveColumns.Keep versus dropping or handling columns not present in the target schema.

6. Show a nested schema or column-group example, since schema conversion is especially relevant for structured DataFrames.

7. Add a troubleshooting section for common failures: missing required columns, unsupported conversion, parse failure, nullability mismatch, and ambiguous custom converters.

8. Clarify when to use convertTo<Schema>() instead of cast<Schema>(), convert { ... }, or manual add/update operations.

Start with the existing Convert and ConvertTo documentation pages and compare their current API examples with the requested APIs, options, and type-conversion behavior. Review the supported-types list and current schema-conversion guidance before updating both pages. Done means the requested examples, distinctions, precedence rules, troubleshooting guidance, and conversion targets are documented accurately.