Move concepts.dd to articles/constraints.dd

[ntrel]

Before it didn't seem to be linked from anywhere on dlang.org. Added redirect rule anyway.
Add link to constraints article from Templates Revisited.
Also group Template Comparison article under other template articles.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23612	enhancement	Template constraints article not listed in article index

[ntrel]

BTW this contains no content changes for constraints.dd. I have updated the article in a separate PR: #3487.