[docs] merge error code lists

Documentation

error_code_list.rst and error_code_list2.rst should be merged. The main result of having two pages is that half the time I end up checking the wrong page if I don't know beforehand whether the code is enabled or disabled by default. Instead, that enablement information should be documented in each entry (possibly also whether or not it's enabled by default in strict mode, ideally).

To elaborate a little, perhaps unnecessarily, I assume there are three types of interactions we could be designing the error codes page(s) for:

1. People reading the documentation cover-to-cover, or similar. Having the error codes split up like this is probably useful to them, but I doubt this is a common interaction.
2. People searching for the error codes using a search engine. These people probably get to the right page either way. While this is useful, usually I don't do this and I try to go directly to the error page, because a general search will sometimes give you unhelpful pages that just happened to mention the code
3. People already on the page, using ctrl-f to cross-reference or find entries. I think this is a useful activity and it's greatly stymied by having them split apart on different pages.

While we're at it, the current error_codes.rst could just be a preamble, and all three pages could be merged into one.

Ideally, preservative redirects would be made. As in any documentation refactor.

まず error_code_list.rst、error_code_list2.rst、error_codes.rst を読んで、現在の構成と、エラーコードのエントリで有効化がどのように記述されているかを理解します。各エントリの有効化の詳細を含む単一の統合ページを計画し、利用可能な場合は strict-mode の状態も含め、既存のページからのリダイレクトを維持します。3つのページが情報を失わずに統合され、古いURLが適切にリダイレクトされれば完了です。