creating release tag v0.7.

Relevant changes (since v0.7):
- Add MacOS support for the test suite and examples (use `getentropy()`
  instead of `getrandom()` and remove `-fsanitize=leak`).
- Rename `SHA3_BACKEND` make argument to `BACKEND` and add support to
  makefiles for all examples and tests.
- Experimental Neon backend Improvements (still not enabled by default).
- Refactor (rename `permute()` to `permute_24()`, `permute12()` to
  `permute_12()`, add a dedicated `sha3_xof12_t` type, and so on).
- Add documentation for internal hash and XOF functions and document
  adding a new backend.
- Miscellaneous documentation fixes.
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