Referring back to the sample ci snippets

[sebasrevuelta]

This PR pretends to simplify the Azure snippets in our documentation.
Two main works done:

1. Improve the sample provided in the Sample CI page.
2. Referring back to this sample in the Azure PR comments page.

For the first point:

The sample in our Sample CI page has been improved with:

• No need to declare the env section with the SEMGREP_APP_TOKEN (It must be set in the Semgrep_Variables).
• Added additional comments to clarify the need to indicate the default branch.
• Added a default image with ubuntu-latest, as it could help customers who want to copy/paste the snippet and see it working.

For the second point

The paragraph that explains how to get a PR comment has been improved.
The snippet has been removed, and it now refers back to the sample CI snippet.

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