Add customizations documentation

[mykhailodanilenko]

🚀 Description

This PR adds initial documentation for the fork customization process.

📄 Motivation and Context

#202 #18 #17

📦 Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

✅ Checklist (copied from README)

• Squash your changes into a single clear and thoroughly descriptive commit, split changes into multiple commits only when it contributes to readability
• Reference the GitHub issue that you are contributing on in your commit title or body
• Sign your commits, as this is required by the automated GitHub PR checks
• Ensure that the changes adhere to the project code style and formatting rules by running npx eslint . and npx prettier --check ../ from the ./OwnTube.tv/ directory (without errors/warnings)
• Include links and illustrations in your pull request to make it easy to review
• Request a review by @mykhailodanilenko, @ar9708 and @mblomdahl