docs: set sidebar order for introductory pages

[golopot]

Fixes fastify/website#308

This PR set sidebar position for introductory pages, making them listed before more advanced topics, in the order of:

• Getting-Started.md
• Database.md
• Recommendations.md
• Testing.md

Docusaurus sidebar docs:
https://docusaurus.io/docs/sidebar/autogenerated#doc-item-metadata

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Fdawgs]

Yes on getting-started but tentative on the order of the rest. Needs a think.

[Eomm]

We have these Project Boundaries
fastify/website#32

We don't want site related stuff here.

We have a many scripts to manipulate the documentation payload:
https://github.com/fastify/website/blob/main/scripts/build-website.sh#L29-L31
So we must implement something there

[jsumners]

We have these Project Boundaries fastify/website#32

We don't want site related stuff here.

We have a many scripts to manipulate the documentation payload: fastify/website@main/scripts/build-website.sh#L29-L31 So we must implement something there

I wanted to ask: is there not a better way to do this that doesn't break our Markdown? Thank you.

[golopot]

We have these Project Boundaries fastify/website#32

We don't want site related stuff here.

I moved the position metadata to the website build script.

This PR is moved to fastify/website#309 .