docs: show conversion from duration to seconds (#44)

## Description
<!-- Please provide a clear and concise description of what the pull
request achieves. Mention the issue it addresses (if applicable). -->

I use Go templates to produce machine-readable files, and one of the
challenges I face is producing amounts of time as seconds and
milliseconds. I want to write values like "5 minutes" or "30 seconds"
for the benefit of file maintainers and have a computer do the math to
turn them into numbers.

The `toDuration` function satisfies my use case and seems valuable to
show in the documentation.

## Changes
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- Detail the changes you have introduced.
- Highlight any new functionality.
-->

One more example for the `toDuration` function.

## Fixes <!-- #issueNumber -->

## Checklist
- [x] I have read the **CONTRIBUTING.md** document.
- [x] My code follows the code style of this project.
- [x] I have added tests to cover my changes.
- [x] All new and existing tests passed.
- [x] I have updated the documentation accordingly.
- [x] This change requires a change to the documentation on the website.

## Additional Information
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This example matches this test:
https://github.com/go-sprout/sprout/blob/aca5c32cd0f8340e64998aa718dee2b3564717d4/conversion_functions_test.go#L130

Author: cbandy

docs/functions/type-conversions.md

@@ -205,6 +205,7 @@ Valid time units are `ns`, `us` (or `µs`), `ms`, `s`, `m` and `h`.
 {{ (1000.0 * 1000.0) | toDuration }} // Output: 1ms
 {{ "1m" | toDuration }} // Output: 1m
 {{ "invalid" | toDuration }} // Output: 0s
+{{ (toDuration "1h30m").Seconds }} // Output: 5400
 ```
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