Document partitioning large NUnit test suites

[Copilot]

Large suites need individually targetable partitions so they can run across multiple jobs. The new how-to explains partition selection and fixture-level grouping.

• How-to: Adds guidance on one-based partition numbering, hash-based assignment, stable partition counts, and relevant caveats.
• Runner examples: Shows Test Selection Language filters for the console runner and dotnet test.

nunit3-console MyTests.dll --where "partition == 2/5"
nunit3-console MyTests.dll --where "partition == 2/5:fixture"

Adds the article to the Writing Tests table of contents.

• Fixes Document how a large suite can be partitioned into individually targetable buckets #778

[stevenaw]

Move it under "docs/docs/articles/nunit/running-tests" instead.
Update command line args here too:

• https://github.com/nunit/docs/blob/master/docs/articles/nunit/running-tests/Console-Command-Line.md
• https://github.com/nunit/docs/blob/master/docs/articles/nunit/running-tests/NUnitLite-Options.md

[Copilot]

Move it under "docs/docs/articles/nunit/running-tests" instead.
Update command line args here too:

• https://github.com/nunit/docs/blob/master/docs/articles/nunit/running-tests/Console-Command-Line.md...

Moved the guide under docs/articles/nunit/running-tests and updated both command-line --where references. Commit: fe50898.

[stevenaw]

Move changes from "Console-Command-Line.md" to "Test-Selection-Language.md"

For all changes, note which version it was added in

• Basic partitioning = NUnit 3.14
• Fixture partitioning = NUnit 5.1
• Console Runner support = NUnit Console 3.17
• Adapter + dotnet test support = NUnit3TestAdapter 4.6

[Copilot]

Move changes from "Console-Command-Line.md" to "Test-Selection-Language.md"

For all changes, note which version it was added in...

Moved the partition filter details from the Console option entry into Test Selection Language and documented the NUnit 3.14, NUnit 5.1, Console Runner 3.17, and NUnit3TestAdapter 4.6 support versions. Commit: 75a7a04.