build: fix changelog and release notes templates after psr update

[afuetterer]

• changelog header is correct
• changelog line length
• changelog compare url per release is working
• unreleased section is just a comparison url
• not too many blank lines between commit types
• there is only one blank line before a new release, not two
• add insertion_flag
• changelog mode is update
• rewrite changelog once with new templates (changelog.mode = "init")
• updating an existing changelog works, without printing "unreleased" twice
• update release note for 0.10
• update release notes template
  • use ctx in release notes template
• update zenodo description for 0.10
• add ref to templates: source is psr templates

Ref:

• https://python-semantic-release.readthedocs.io/en/stable/changelog_templates.html
• https://github.com/python-semantic-release/python-semantic-release/tree/master/src/semantic_release/data/templates/angular/md