doc: add documentation of environment variables (#762)

* doc: add documentation of environment variables

* doc: add the English version and modify README.md

* doc: remove descriptions of env vars for cobj

* doc: fix documents

* doc: fix errors detected by Copilot

* doc: remove `Usage` sections

Author: yutaro-sakamoto

README.md

@@ -172,6 +172,7 @@ java [PROGRAM-ID]
 * [opensource COBOL 4J: Java変換解説](./doc/converted_Java_file_JP.md)
 * [configuration-params-JP](./doc/configuration-params-JP.md)
 * [The specification of locking for INDEXED files in opensource COBOL 4J](./doc/specification-locking-indexed-file.md)
+* [Environment Variables Reference](./doc/environment_variables.md)
 
 ## Development Progress
 

README_JP.md

@@ -161,6 +161,7 @@ java [PROGRAM-ID]
 * [opensource COBOL 4J: Java変換解説](./doc/converted_Java_file_JP.md)
 * [configuration-params-JP](./doc/configuration-params-JP.md)
 * [INDEXEDファイルのロックの仕様](./doc/specification-locking-indexed-file_JP.md)
+* [環境変数リファレンス](./doc/environment_variables_JP.md)
 
 ## 実装状況
 実装済み機能：