…(), and is_equivalent certificate

- Removed greek letter alpha in docstrings in hopes of this fixing infinite build loops
- Updated `.is_definite()` to fit PR sagemath#37173 (up to the reference to Voight's book)
- Other small modifications of docstrings, comments and error warnings
- Slightly cleaned up code with respect to intermediately defined variables

- Added deprecation warning for `.is_equivalent`
- Removed unnecessary call of `.is_positive_definite()` in `.reduced_basis()`
- Reduced `.is_right_equivalent` to `.is_principal` with suitable rescaling
- Fixed some error warnings
- Modified docstrings, moved examples to non-deprecated methods and randomized test of `.is_principal`

- Modified definiteness check in `.minimal_element()` and `.isomorphism_to`; also added check of the base field in the latter method
- Reduced search for a generator in `.is_principal()` to a call to `.minimal_element()`

This is not a fix for the issue - it just adapts the error warning to be more ambiguous, as suggested in sagemath#34976 (comment)