Add tutorial on creating Custom Media Backends

[airtonix]

Describe the bug

documentation is missing a guide on creating our own media backend

To Reproduce

1. Go to https://decapcms.org/docs
2. Scroll to Media
3. See nothing regarding "Creating your own media backend"
4. Sigh in frustration.

Expected behavior

No sighing in frustration.

Screenshots

./picture of me sighing in frustration

Applicable Versions:

n/a

CMS configuration

n/a

Additional context

should probably have dedicated api section instead of mixing up information for "editors" and developers, current format is confusing and not predictable to find information.

- Intro
- Get Started
- Editors
  - Intro
  - Publishing workflow
  - Help where is my page i just saved?
  - OMG Y R IT SO????
  - ...
  - Working with images
- Developers
  - Installation
  - ...
  - Guides
- Api
  - Collections
  - Widgets
  - MediaBackends
  - etc

[airtonix]

Another use case is integrating https://fengyuanchen.github.io/compressorjs/ after the user picks the image and before we upload to the backend

[privatemaker]

@airtonix this is really good point. I created a collection of self-hosted media tools to investigate towards this goal.