docs: add example section for HmacSecretKey class

[HamdaanAliQuatil]

• [x] Added Examples section for the HmacSecretKey class in the API Reference Docs
• [x] Made the sample code self-contained which one could copy and run.

Here's the self-contained code and the terminal logs: https://gist.github.com/HamdaanAliQuatil/bd32945af3091ff7ba72071386ff4de7

[HamdaanAliQuatil]

Takes a step towards #101

[jonasfj]

The comment about length is a little confusing. The rest looks fine, my comments are just nits :)