Add a proper changelog

[Eric-Arellano]

Right now, we do ad-hoc changelogs in GitHub release notes:

qiskit_sphinx_theme/CONTRIBUTING.md

Lines 56 to 59 in 29277ce

	2. Add release notes, e.g. https://github.com/Qiskit/qiskit_sphinx_theme/releases/tag/1.11.0rc1
	1. Add the sections `**Features / API Changes:**` and `**Bug Fixes:**`.
	2. Use `git log --oneline` to see what changes have been made. Copy and paste those entries as bullets into the relevant sections. Ignore any "internal only" changes like CI changes or updates to the README.
	3. Preview the release notes with the "Preview" tab.

It would be better to also have a proper changelog file in the repo. A main benefit is centralizing all the notes in one single place.

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We probably do not need to use Reno for changelog generation. This repo is small and simple enough that manually maintaining a Markdown file is reasonable. Likely we should have PR authors update the changelog when making a change.

Relates to #214 - we need to figure out how to backport the changelog when backporting bug fixes.

[Eric-Arellano]

Going to close as low priority since this repo gets little traffic and few releases. Still a good idea to implement though if anyone wants to take it on.