Suggestion on re-arranging the docs

[namelessvoid]

This PR is a suggestion for #259 .

Updates to the structure:

• The "Getting Started" section contains installation instructions for GDExtension version, module version and on C#.
• The "Behavior Trees" and "Hierarchical State Machines" sections contain their respective docs.
• The "Important classes" page was moved to the top of the "Class Reference" section.

I did not remove any info (just moved it - if something is missing, this was unintentional). I added some lines on HSMs on the index page and also added some "filler" text.

The proposed structure looks like this:

[namelessvoid]

Note: I did not update the Readme file yet. If you like my proposal, I can update the Readme, too 🙂

[limbonaut]

There's also "Contributing" page missing that was added recently. You may want to rebase your branch to see it.

[namelessvoid]

There's also "Contributing" page missing that was added recently. You may want to rebase your branch to see it.

True! I did rebase yesterday, but forgot to update the screenshot. The Contributing page is located in the "Getting Started" section.

[limbonaut]

Looks good! I just have a few suggestions before we merge it.

[namelessvoid]

I like your suggestions and applied them! The Links in the Readme are now updated as well.

[limbonaut]

Thanks