Add `README.md` to all tools

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.

- [x] `code-validation` (#1052)
- [x] `generate-assets` (#1052)
- [ ] `generate-community`
- [x] `generate-errors` (#1089)
- [ ] `generate-release`
- [ ] `generate-wasm-examples`
- [x] `write-rustdoc-hide-lines`

Compare the existing README.md files in code-validation, generate-assets, and generate-errors, then inspect the generate-community, generate-release, and generate-wasm-examples directories. Document what each tool does and how to use it; the work is done when all three unchecked tool directories contain README.md files with that information.

Written by the indexing model from the issue text.