Improve infrastructure for our documentation

[wolfgangwalther]

This collection of postgrest-docs issues is kind of a "meta issue", because many of the issues are about improving the docs themselves, not about specific pieces of content:

• Query Grammer postgrest-docs#228
• Provide sample db for running the references postgrest-docs#330
• Add an Under the Hood page postgrest-docs#348
• Automated testing for tutorials postgrest-docs#371
• Add demo server postgrest-docs#448
• Include generated query in the API postgrest-docs#528
• Dedicated landing page postgrest-docs#628

Another suggestion is to add a FAQ:

• Add FAQ postgrest-docs#272

This FAQ could possibly include:

• GET call with in filter to POST call (414 URI too long) postgrest-docs#417
• Should we document risks with column names longer than NAMEDATALEN? postgrest-docs#471
• Add snippet for reloading on read replicas postgrest-docs#638