Add frontmatter validations

[fabianrbz]

Schemas for each content_type are defined in app/_data/schemas/frontmatter/.
These are basic validations, the idea is to add more as we write docs or find new validations.

A tool that validates the frontmatter (except for plugin pages and autogenerated content for now) lives in tools/frontmatter-validator and a GH action runs it as a check.

Considerations:

• title and content_type are required for all pages
• description isn't required for how-tos for now, they have a tldr
• reference pages require products or tools
• how_tos require tldr, works_on if products == gateway

Note:

• It outputs some warnings, I think the problem is that ajv - which is the tool that we use to load the json-schemas and validate the date - runs in strict mode and has some weird rules defined.
• There are currently 46 errors. All of them are because those pages don't have a description, I need help fixing those.

How to run it

1. cd tools/frontmatter-validator
2. npm ci
3. node index.js

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