Add link to main branch in versioned docs (#246)

Author: dduan

Sources/TOMLDecoder/TOMLDecoder.docc/CHANGELOG.md

@@ -45,7 +45,7 @@ and convert these initial value types into compatible types in the `Decodable`s.
 ### Infrastructure improvements
 
 * The full toml-lang/toml-test suite is now imported as unit tests.
-* [A new documentation site](https://dduan.github.io/TOMLDecoder/documentation/tomldecoder/).
+* [A new documentation site](https://dduan.github.io/TOMLDecoder/0.4.x/documentation/tomldecoder/).
 * More comprehensive CI jobs.
 * New benchmark infra, and suite.
 

Sources/TOMLDecoder/TOMLDecoder.docc/VersionedDocs.md

@@ -2,7 +2,8 @@
 
 Links to docs for each TOMLDecoder releases.
 
-* [Version 0.4.0](https://dduan.github.io/TOMLDecoder/0.4.x/documentation/tomldecoder/)
+* [the main branch](https://dduan.github.io/TOMLDecoder/main/documentation/tomldecoder/)
+* [Version 0.4.0](https://dduan.github.io/TOMLDecoder/0.4.x/documentation/tomldecoder/) (latest release)
 * [Version 0.3.x](https://github.com/dduan/TOMLDecoder/tree/0.3.3/Documentation)
 * [Version 0.2.x](https://github.com/dduan/TOMLDecoder/tree/0.2.2/Documentation)
 * [Version 0.1.x](https://github.com/dduan/TOMLDecoder/tree/0.1.6/Documentation)