documentation structure overhaul (#1602)

## Motivation for the change, related issues

This PR is related to the issue
#1518

Its goal to provide a better structure for WordPress Playground Docs and
a better learning journey for those navigating these docs

[<img width="1677" alt="Screenshot 2024-08-05 at 13 11 52"
src="https://github.com/user-attachments/assets/22b03afd-921a-4584-843d-0b452302496f">
](https://youtu.be/gzrW4Pht1Fc)
[See video with overhaul proposal](https://youtu.be/gzrW4Pht1Fc) 

<details>
<summary>See Video of initial version of this PR</summary>

https://github.com/WordPress/wordpress-playground/assets/422576/afdda902-e50c-4d72-b263-5a11323d1282
</details>

## Implementation details

This PR creates three different subsites to organize three big types of
docs related to WP Playground:
- **Main**: Introduction to WP Playground, First steps, and more
End-User oriented
- **Blueprints**: A specific section for this topic that is in the
middle ground between non-technical and technical users
- **Developers**: All WP playground-related info relevant for developers
with a high-level techie language

For that, three dedicated sidebars (for each one of these subsites) have
been created. All markdown files have been reorganized across three
folders and links have been updated accordingly

## Testing Instructions (or ideally a Blueprint)

- checkout this branch
- from the root of the project
  - run `npm run build:docs`
  - run `npm run dev:docs`
  
## To-Do
- [x] Add Filesystem and DB debug info as per
#1533
-
#1533 (comment)
- [x] #1529 Take
these SEO suggestions into account for final PR - no clear actions for
the docs at the moment - [awaiting for more
feedback](#1529 (comment))
- [ ] ~The breadcrumb on https://wordpress.org/playground/ reads
“Playground”, can we please make that “WordPress playground”?~
- [ ] ~Can we change the sentence “The platform that lets you run
WordPress instantly on any device without a host.” to “WordPress
Playground is the platform that lets you run WordPress instantly on any
device without a host.”~
- [ ] ~Change the sentence “A single <iframe> tag is all it takes to
integrate Playground with your application.” to “A single <iframe> tag
is all it takes to integrate the WordPress Playground with your
application.”~
- [ ] ~A small social improvement - We should really give the WP
Playground “homepage” an og:image~

---------

Co-authored-by: Adam Zieliński <[email protected]>