Update documentation for v0.21 deprecations

[silverlyra]

👋🏻 Hello! I used this base64 crate for the first time this past week in a project. Honestly when I started, I first didn’t even look at the documentation – I ran a cargo add base64, and then tried typing base64::decode in my editor.

That function did exist, and does exactly what I wanted (decode canonical base64 input using the standard alphabet), but it showed as deprecated. So I went to docs.rs to see what I was supposed to use instead, and I found the current crate documentation difficult to understand how to set up base64 decoding and/or encoding.

I appreciate this crate and the flexibility it offers for the nuances of base64 (different alphabets, padding = character handling), and here I’ve made an attempt to update the crate documentation to help a new user navigate those nuances, and successfully start decoding/encoding data.

In the updated documentation, I’ve made sure there’s a code sample immediately visible which shows how to do decoding and encoding using default settings, and I (attempt to) explain enough of the nuance of base64 to empower users who would want non-default choices to get the setup they need.

I’ve submitted many more pull requests on code than on prose 😅 – I’m not sure how you’d want to review this, but I can keep workshopping the text. 🙏🏻 I also made some tweaks to the existing base64.rs example.

[marshallpierce]

Thanks very much for the thoughtful improvements! I've taken care of the conflicts (#263) and tweaked a couple of minor things (notably, not renaming the engine imports to BASE64 to highlight to the hasty reader that they are in fact different).