Use placeholder for release versions before release

[jl-wynen]

When adding directives like .. versionadded::, we need to provide a version number. But this is impossible to predict in CalVer.
So use a placeholder version. And implement a check in the release workflow to look for placeholders and stop if it finds any. Also add a script to replace placeholders. (This should not run automatically to reduce the risk of breaking the code.)

In Scipp, we use PLACEHOLDER or vPLACEHOLDER. Is this good enough or should we use a different placeholder text?

[YooSunYoung]

I just used RELEASE_PLACEHOLDER in #153

Cause it looked like prepare-release tox command uses this in scipp...?

[testenv:prepare-release]
description = Run before a release. Pass the release version as a positional argument.
skip_install = true
deps =
allowlist_externals = sh
commands =
  sh -c 'git grep -rl RELEASE_PLACEHOLDER docs/ src/scipp | xargs sed -i "s/RELEASE_PLACEHOLDER/{posargs}/g"'

[jl-wynen]

See also scipp/copier_template#35