Close 2.3.1: date the changelog and ship the analyzer release #254

Release steps 2 and 3 of CONTRIBUTING's "Releasing" for 2.3.1, and the version named where step 1 says:

• CHANGELOG.md: the 2.3.1 section dated, its links pointed at blob/v2.3.1/.
• AnalyzerReleases.Shipped.md: ## Release 2.3.1 in place of the empty ## Release 2.3.0. 2.3.1 adds no rule (CMTK0005 and CMTK0006 widen rules shipped earlier), so Unshipped.md is already empty.
• docs/comparison.md and the root README's Status name 2.3.1. No TypeKit cell of the comparison changes: 2.3.1's fixes touch none of them.

Step 1 ran locally on release/2.3.1 at bce9e84: docs-query due --nuget lists no fact due (0 of 34), and oracles seeds caught every seed, every baseline passing, the SQL Server oracles included (exit 0).