Improve the documentation for obtaining central token and updating upon token expiration

[Thevakumar-Luheerathan]

Description

https://ballerina.io/learn/publish-packages-to-ballerina-central/#obtain-an-access-token
In this guide, it shows how to obtain the token, but there's no information about token expiration and how to update it when it expires. (Or whether it expires at all).

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