Add option to siteConfig.js to configure supported headers in front matter

[avishnyakov]

💥 Proposal

Add SupportedHeaderFields option or similar to siteConfig.js so that we can avoid warnings about unsupported header fields.

The use case is this one:

• We store additional metadata in markdown front matter section
• Docusaurus reports warning on "unsupported" header fields as per "Ignore and warn about unsupported header fields" Ignore and warn about unsupported header fields #347

A suggestion is to move SupportedHeaderFields set into siteConfig.js defaulting/merging current set with the user provides. Optionally, we might also throw an exception if front matter attribute does not fit supported header field set. Such behaviour is useful on a large set of markdown files; catching non-standard front matter, typos in header names, and removing warnings from the builds.

Have you read the Contributing Guidelines on issues?

Yes