Add `README.md` to all tools

[BD103]

There are multiple tools in this repository used for generating the site. Unfortunately, not all of them are well documented. A good step in this direction is to add a README.md file to each directory, documenting what the tool does and how to use it.

• code-validation (Add READMEs to code-validation and generate-assets #1052)
• generate-assets (Add READMEs to code-validation and generate-assets #1052)
• generate-community
• generate-errors (Rewrite generate-errors and improve learn/errors #1089)
• generate-release
• generate-wasm-examples
• write-rustdoc-hide-lines