mathlib3

b62b5314 - doc(analysis/normed_space/basic): Explain how to use non-unital normed algebras (#13605)

doc(analysis/normed_space/basic): Explain how to use non-unital normed algebras (#13605) [Zulip](https://leanprover.zulipchat.com/#narrow/stream/116395-maths/topic/.E2.9C.94.20Is.20the.20zero.20algebra.20normed.3F/near/279555566)