docs/explanation/security: modified snapd security documentation to be SEC30 V1.3 compliant

[ernestl]

Follow up of WIP #179 that was accidentally merged early.

Please also look at provide feedback on #179.

Spec: SEC30

Jira: https://warthogs.atlassian.net/browse/SNAPDENG-35755

[pedronis]

did a first pass

[Copilot]

Pull request overview

This PR extends the Sphinx/MyST security documentation by adding new pages/sections covering snapd API authentication/authorization and snapd decommissioning, and by expanding existing security policy documentation (including cryptography provenance and security maintenance lifecycle details). The PR description references a follow-up to PR #179, but this review is limited to the diffs included here.

Changes:

• Add new “API authentication and authorization” and “Decommissioning” explanation pages and link them from the security index.
• Expand security-policies.md with cryptography provenance notes and a new “Security maintenance” section; fix a few formatting issues in refresh awareness text.
• Add a dedicated anchor in data-locations.md for “Persisted data on Ubuntu Core” and update cross-references to point to it.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
docs/reference/administration/data-locations.md	Adds an anchor label for the Ubuntu Core persisted data section to support more specific cross-references.
docs/explanation/security/snap-confinement.md	Adjusts the Classic confinement link (currently in a way that likely breaks the reference).
docs/explanation/security/security-policies.md	Adds cryptography provenance + security maintenance content; updates cross-references and refresh awareness formatting.
docs/explanation/security/index.md	Adds nav links/toctree entries for the new security pages (currently with a toctree filename mismatch).
docs/explanation/security/decomissioning.md	Introduces decommissioning guidance (currently has label whitespace + typos + structure issues, and filename spelling mismatch).
docs/explanation/security/api-authentication-and-authorization.md	Introduces API auth/authz overview content, including auth data location and authorization mechanisms.

[bboozzoo]

LGTM, assuming comments from copilot are addressed

[pedronis]

one comment, need to do a pass still on api-auth-authz

[pedronis]

comment, let me know how you want to proceed on this, picking different examples, pointing to other docs?

[rmoragas]

This is actually very good. We'll start covering diagrams (architecture) and more detailed explanation in 26.10/27.04, so this is definitely going to be useful.

[pedronis]

I'm not against the diagrams but the 2nd one needs a tweak

[ernestl]

I did fairly heavy modification to authentication and authorization to try and have it more accurate.
Also the diagrams was modified.

[ernestl]

Authentication:

[ernestl]

Authorization:

Note - zooming is enabled in grahams official version of mermaid support, and scale and style can be tweaked in the future.

[pedronis]

generally +1, please get Maciej to re-review if there are major changes

[ernestl]

Maciek did complete another pass.