Expedite removal convention parameter in from/to_euler() methods

Description of the change

This PR changes the planned removal of the convention parameter in the class methods to convert between quaternions and Euler angles to v0.13, in two minor releases. The original plan was to remove it in v1.0, but there are many kinks left to iron out before that, so I suggest to remove it in v0.13 as it was deprecated since v0.9.

I've made some other minor changes to wording and simplified some code in the docs as well.

These changes were originally added in for #442, but since that PR has to wait, I've made this PR instead.

Progress of the PR

[n/a] Docstrings for all functions
[n/a] Unit tests with pytest for all lines
[x] Clean code style by running black via pre-commit

Minimal example of the bug fix or new feature

See the docs built from this PR: https://orix--446.org.readthedocs.build/en/446/changelog.html.

For reviewers

[ ] The PR title is short, concise, and will make sense 1 year later.
[ ] New functions are imported in corresponding __init__.py.
[ ] New features, API changes, and deprecations are mentioned in the unreleased section in CHANGELOG.rst.
[ ] Contributor(s) are listed correctly in __credits__ in orix/__init__.py and in .zenodo.json.

pyxem / orix