Fix links to sigstore/docs in markdown files (#3064)

The links both to the docs repo but also the rendered ones were broken.
Let's replace them with ones that do work as of today. The sign.md
document no longer exists in the sigstore/docs repo, but the overview
one is probably the closest in intent.

Signed-off-by: Jakub Hrozek <jakub.hrozek@posteo.se>

EXAMPLES.md

@@ -1 +1 @@
-> Note of deprecation: This document has been migrated and merged into into [`sigstore/docs`](https://github.com/sigstore/docs/blob/main/content/en/cosign/sign.md) as part of [documentation migration](https://github.com/sigstore/cosign/issues/822) and PR: https://github.com/sigstore/docs/pull/123. To view the live docs page, go to: https://docs.sigstore.dev/cosign/sign/#generate-the-signature-payload-with-cosign-to-sign-with-another-tool
+> Note of deprecation: This document has been migrated and merged into into [`sigstore/docs`](https://github.com/sigstore/docs/blob/main/content/en/cosign/overview.md) as part of [documentation migration](https://github.com/sigstore/cosign/issues/822) and PR: https://github.com/sigstore/docs/pull/123. To view the live docs page, go to: https://docs.sigstore.dev/cosign/overview/

KEYLESS.md

@@ -1 +1 @@
-> Note of deprecation: This document has been migrated into [`sigstore/docs`](https://github.com/sigstore/docs/blob/main/content/en/cosign/keyless.md) as part of [documentation migration](https://github.com/sigstore/cosign/issues/822) and PR: https://github.com/sigstore/docs/pull/56. To view the live docs page, go to: https://docs.sigstore.dev/cosign/keyless
+> Note of deprecation: This document has been migrated into [`sigstore/docs`](https://github.com/sigstore/docs/blob/main/content/en/cosign/openid_signing.md) as part of [documentation migration](https://github.com/sigstore/cosign/issues/822) and PR: https://github.com/sigstore/docs/pull/56. To view the live docs page, go to: https://docs.sigstore.dev/cosign/openid_signing/

USAGE.md

@@ -1 +1 @@
-> Note of deprecation: This document has been migrated and merged into into [`sigstore/docs/sign`](https://github.com/sigstore/docs/blob/main/content/en/cosign/sign.md) and [`sigstore/docs/verify`](https://github.com/sigstore/docs/blob/main/content/en/cosign/sign.md) as part of [documentation migration](https://github.com/sigstore/cosign/issues/822). To view the live docs page, go to: https://docs.sigstore.dev/cosign/sign and https://docs.sigstore.dev/cosign/verify
+> Note of deprecation: This document has been migrated and merged into into [`sigstore/docs`](https://github.com/sigstore/docs/blob/main/content/en/cosign/overview.md) as part of [documentation migration](https://github.com/sigstore/cosign/issues/822) and PR: https://github.com/sigstore/docs/pull/123. To view the live docs page, go to: https://docs.sigstore.dev/cosign/overview/