Make templates self describing

# Summary
It would be great workflow template had a description and parameters had a description property. This would allow `argo template get ` to produce output more like the help of a regular terminal command.

For example current output is:
```
> argo template get my-template
Name: my-template
Namespace: ns
Created: Wed Sep 22 15:05:32 +0200 (3 weeks ago)
```
but it would be great if it output something
```
> argo template get my-template
Name: my-template
Namespace: ns
Created: Wed Sep 22 15:05:32 +0200 (3 weeks ago)

Description:
Send an email to head office and make some tea.

Usage:
argo submit my-template -p arg1= -p arg2=

Parameters:
arg1 Description of arg1
arg2 Description of arg2 (default: 5)
```
# Use Cases

When creating templates for other team members to use.

---

**Message from the maintainers**:

Impacted by this bug? Give it a 👍. We prioritise the issues with the most 👍.

Start by tracing the `argo template get ` and `argo submit` entry points and how workflow templates and parameters are represented. Done means template output includes the workflow description, a submission usage line, and each parameter's description and default value where applicable.

Written by the indexing model from the issue text.