Changelog: a released version's links point at blob/main, not its tag #76

Problem

Every line of CHANGELOG.md links its docs at blob/main (…/blob/main/src/CodoMetis.TypeKit.EntityFrameworkCore/README.md#queries). Since #75 the version's section is each package's release notes on nuget.org, frozen with the version, so a reader of 2.3.0's notes who follows a link a year later lands on that day's README, where the section may say something else or the anchor may be gone. It is the problem #75 fixed for the changelog link itself, one level down. The GitHub release's notes have the same links.

Change

Point a released version's links at its tag: blob/v<version>/ in place of blob/main/. Either when the release is prepared (CONTRIBUTING, Releasing, step 1, where the heading is dated), or at pack for a dated section only, since an Unreleased pack has no tag yet. The release workflow's GitHub notes should read the same text. A guard holds a dated section to links at its own tag.

Done when

A dated section's links, in CHANGELOG.md, the packed nuspecs and the GitHub release, name that version's tag, and a guard fails on a blob/main link in one.