Docs updates

[yanmastin-astro]

Description

The purpose of this PR is to improve the docs in the Astro terraform registry listing.

The files in the docs folder are autogenerated from default templates by the tfplugindocs tool during the build. You can overwrite the default templates using .tmpl files in the templates folder which is added in this PR. You can also add .md files into the templates/guides folder to add markdown formatted user guides.

This PR:

• Improves the Astro Terraform Provider listing doc homepage by adding a index.md.tmpl file that has high level descriptions, authentication information, in addition to the existing autogenerated Example Usage and Schema sections.
• Adds a get-started.md guide that walks through a simple example use case of Astro Terraform Provider.
• Adds a import-script.md guide that shows how to use the import script to automate the migration of existing resources into Terraform.

[vandyliu]

few comments but thanks for doing this!
the docs look much cleaner now!!

[ichung08]

thanks for adding more steps to the guide

i think the overall structure is better however it would also be helpful for the guide to be written in a more step by step guide (similar to the get-started.md one) where you could take the user through the process of creating an org, workspace, deployment, api-token (just a couple resources) in the UI and then walk them through the process of setting up and running the import script. the way it is right now also works, but i think it would be beneficial for this to be a more interactive experience

also you'll notice i have a couple comments on some odd formatting, i think your editor formats < and > differently

[yanmastin-astro]

Moved content to #159