docs: add 0.6.1 changelog entries

[utiberious]

Summary

Adds changelog entries for 0.6.1, covering all PRs merged since 0.6.0:

• feat feat(lt): implement Mlt component-expression constructor #580 — Mlt component-expression constructor (closes Mlt: implement component-expression constructor (currently NotImplementedError) #578)
• bug Multivector method .is_blade() will fail for null multivectors #537 — is_blade() null multivector fix (pending PR)
• support docs(macdonald): update GAlgebraPrimer.pdf to September 2023 version #577 — Macdonald GAlgebraPrimer.pdf update (closes Sync bundled Macdonald materials with upstream (GAlgebraPrimer Sep 2023) #575)
• support docs: update README Python prereqs; add dev guides for bumping Python and SymPy (#571) #572/docs(dev): add release process runbook (#573) #574 — dev guides for bumping Python/SymPy, release runbook
• support fix(notebooks): refresh outputs for SymPy 1.13 printing changes (#568) #570 — SymPy 1.13 notebook refresh (closes Update notebook outputs for SymPy 1.13 printing changes #568)
• bug fix(lt): accept ImmutableDenseMatrix in Lt.__init__ (#567) #569 — Lt ImmutableDenseMatrix fix (closes Lt does not accept ImmutableDenseMatrix from SymPy 1.13 #567)
• support ci: bump Python to 3.10-3.12, drop 3.8/3.9 (#564) #565 — Python 3.10–3.12 CI bump (closes Bump Python version: drop 3.8/3.9, add 3.12 #564)

Note: the is_blade entry (#537) anticipates the fix in the companion PR — can be merged after that PR lands, or the entry can be moved to a later release if the fix isn't ready.

[utensil]

Thanks for putting this together. One small fix: the is_blade entry uses :bug:537 (the issue number) but all other entries use PR numbers — it should be `:bug:`582.

[utensil]

Thanks, merging.