📖 updated extension generator markdown template

[CrystalOnScript]

This PR updates the template used to create reference documentation for new amp components.

[rsimha]

LGTM overall. I'll let other reviewers approve the contents.

Can you sanity check this to make sure the extension generator doesn't conflict with prettier?

# Generate an extension
gulp make-extension --name amp-crystal

# Make sure there are no formatting errors
gulp prettify --files "extensions/amp-crystal/**.md"

If all goes well, the second step should not yield any formatting errors.

If there are errors, you'll have to either adjust extension-doc.template.md, or modify this code:

amphtml/build-system/tasks/extension-generator/index.js

Lines 76 to 81 in 185493b

	const getMarkdownDocFile = async name =>
	(await fs.readFile(`${__dirname}/extension-doc.template.md`))
	.toString('utf-8')
	.replace(/\\\$category/g, '$category')
	.replace(/\\?\${name}/g, name)
	.replace(/\\?\${year}/g, year);

[sebastianbenz]

Left a few comments. I think it'd be good to simplify the structure:

• only once include an executable example template. If it makes sense, mention it in the section description instead.
• explicitly mark all optional sections

[rsimha]

Thanks for all the due diligence!

[sebastianbenz]

nit: wrap the attribute names in `

[sebastianbenz]

nit: wrap the action names in `

[sebastianbenz]

should the value be wrappen in `?

[sebastianbenz]

Few nits left. Otherwise LGTM

[sebastianbenz]

add line break before filter