reuse: add copyright + license info to individual docs/*.md files

Instead of use 'docs/*.md' in dep5. For clarity and avoiding a wide-
matching wildcard.

+ Remove mention of old

reuse: add copyright + license info to individual docs/*.md files

Instead of use 'docs/*.md' in dep5. For clarity and avoiding a wide-
matching wildcard.

+ Remove mention of old files from .reuse/dep5
+ add info to .github/dependabot.yml
+ make scripts/copyright.pl warn on non-matching patterns

Closes #13245

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CI: Add dependabot.yml

This will cause dependabot to open a PR when various actions are
updated, provided that the action maintainer has issued a release.

Closes #12623