Bump to 6.2.0-alpha1 and start docs/NEXT-RELEASE.md (#3014)

* Bump to 6.2.0-alpha1 and start docs/NEXT-RELEASE.md

Release notes for the version in development live in the repo now, so they are
versioned with the change that caused them instead of being reconstructed from
the commit log at the end. Every PR that changes something a user can notice
adds its entry to docs/NEXT-RELEASE.md in the same commit, the file grows
through the alphas, and it is copied into the release body as it is.

Filled it with the notes for 6.2.0-alpha1, covering everything since the 6.1.0
tag: the Pester.BeforeContainer.ps1 cascade, the Should-BeString region diff,
strict configuration values, error locations, and parallel on Windows
PowerShell 5.1.

ModuleVersion 6.2.0, Prerelease alpha1, RequiredAssemblyVersion 6.2.0, and the
ReleaseNotes URL points at the 6.2.0-alpha1 tag.

🤖

* Apply review comments on the 6.2.0-alpha1 notes

Drop the "Pester 6 runs on Windows PowerShell 5.1 and PowerShell 7.4+" line, nothing
changed there and it reads as if something did. Say the root setup file holds the common
setups and teardowns every test needs, it only ever applies to tests. Describe multiple
BeforeAll/AfterAll per block as something you can now do, with an example, instead of as
an error that stopped happening. Remove the Repository section, CI and the self-test
tracer are not something a user of the module has any use for.

Also rewrote the maintainer comment at the top to say what the file is actually for: the
notes are written once for the first alpha, which is usually close to feature complete, so
there is one set to read and approve and every prerelease after it is a small diff.

🤖

Author: nohwnd