creating release tag v0.6.

Relevant changes (since v0.5):
* Improve speed of scalar `pi()` and `chi()` with shared temporary array.
* Improve absorb and squeeze speed by absorbing and squeezing in chunks.
* Remove `shake{128,256}()` and rename `shake{128,256}_xof_*()` to `shake{128,256}_*()`.
* Add CAVP tests in `tests/cavp-tests`.
* Reduce code length by parameterizing all algorithms, using a common `absorb()` across all algorithms, removing unused internal functions, and simplifying failures in test functions.
* Documentation improvements: Bump copyright year, fix links, fix typos, improve comments in examples, document CAVP tests.
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