docs: add OpenSSL certificate inspection tutorial

[WilliamK112]

Related issue

Closes #875

Description of changes

• add a focused OpenSSL tutorial for inspecting Trustpoint-issued certificates
• show how to extract only the public certificate from a downloaded PKCS#12 credential
• explain Subject, Issuer, validity, public-key algorithm, and signature algorithm
• distinguish content inspection from trust-chain and revocation verification
• add the tutorial to the Getting Started navigation and link it from the credential-download flow

Validation

• generated a temporary certificate and PKCS#12 bundle, then executed both documented OpenSSL commands
• verified the extracted certificate fingerprint matches the original
• rendered the new page and confirmed its navigation and cross-references in generated HTML
• compared strict Sphinx builds of main and this branch: both report the same 41 pre-existing warnings/errors, with no warning from the changed files
• git diff --check

Legal

• I certify that I have all necessary rights to publish this contribution under the MIT license. I agree to the Trustpoint CLA and have added my name to the AUTHORS.md file.

The legal checkbox is intentionally left open until I personally confirm the CLA and author entry.

[FHatCSW]

HI @WilliamK112,
thanks for your contribution. Really like your docs section!
Cheers
Florian

[FHatCSW]

Approved