fix: update context providers docs link

[anhtnt90dev]

Summary

• update the @ dropdown "Add more context providers" action to open the current Context Providers docs page
• replace the old redirected URL that lands on Page not found

Fixes #12507

Test plan

• Verified the old docs URL lands on Page not found
• Verified https://docs.continue.dev/customize/deep-dives/custom-providers loads the Context Providers page
• Ran git diff --check
• Ran git grep for the old and new Continue docs URLs in the changed file

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Summary by cubic

Update the @ dropdown “Add more context providers” action to open the current Context Providers docs, replacing a stale URL that 404s.

^{Written for commit 9df6487. Summary will update on new commits.}

[github-actions]

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[cubic-dev-ai]

No issues found across 1 file

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