update(packaging): make changelog compliant with 'keep a changelog' convention

[Guts]

This PR cleans up the changelog to make it compliant with the https://keepachangelog.com/fr/1.1.0/ convention which is used behind the scenes by a lot of tools and especially QGIS Plugin CI, which was added to this repository back in 2021 (#47).

Here are the main advantages:

• document changes in a conventional file (CHANGELOG.md) instead of changelog section of metadata.txt
• when using QGIS Plugin CI with package or release commands, it will load the CHANGELOG.md content to extract the latest version number (or the one specified), the associated changes and write them into the metadata.txt
• Github understands and features the CHANGELOG.md
• it can be added to the online doc

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• ❤️ Funded by Oslandia, @eptb-loire and ANFSI