[Bug] Fix support for markdown template

[sserrata]

Description

Introduces changes/fixes to allow users to specify a mustache template file for customizing the markdown/MDX structure for API docs. Although it's technically possible to prepend or append content to the ApiItem and ApiDemoPanel sections the primary use case is to customize the MDX frontmatter.

Motivation and Context

Allows users to customize how the API MDX is rendered so they can do things like customize the frontmatter custom_edit_url and other fields.

How Has This Been Tested?

Tested with Petstore API. This PR also includes a sample mustache template and usage.

[github-actions]

Size Change: +12.9 kB (0%)

Total Size: 3.68 MB

Filename	Size	Change
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demo/build/assets/js/09e07b87.********.js	2.92 kB	-58 B (-2%)
demo/build/assets/js/177e86db.********.js	10.8 kB	+581 B (+6%)	🔍
demo/build/assets/js/19d75775.********.js	9.08 kB	+615 B (+7%)	🔍
demo/build/assets/js/39d00f16.********.js	10.4 kB	+580 B (+6%)	🔍
demo/build/assets/js/47abaca6.********.js	2.83 kB	-48 B (-2%)
demo/build/assets/js/4b891ed6.********.js	11.7 kB	+639 B (+6%)	🔍
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demo/build/assets/js/8ab9b16e.********.js	10.1 kB	+531 B (+6%)	🔍
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demo/build/assets/js/e015ccf9.********.js	26 kB	+661 B (+3%)
demo/build/assets/js/eaf4471e.********.js	12.3 kB	+546 B (+5%)	🔍
demo/build/assets/js/f212caf7.********.js	23.2 kB	+562 B (+2%)
demo/build/assets/js/f65290a6.********.js	10.7 kB	+601 B (+6%)	🔍
demo/build/assets/js/main.********.js	392 kB	+34 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
demo/.docusaurus/globalData.json	19.1 kB	0 B
demo/build/assets/css/styles.********.css	92.8 kB	0 B
demo/build/assets/js/006e727e.********.js	13.8 kB	+10 B (0%)
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demo/build/assets/js/runtime~main.********.js	8.71 kB	0 B
demo/build/index.html	80.1 kB	+10 B (0%)

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[github-actions]

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