Extend portfolio access control docs

[nscuro]

• Explicitly calls out that permissions and project access are orthogonal concepts.
• Provides example pattern that demonstrates how per-project permissions can be emulated.
• Documents how ACL is inherited, may be bypassed, and behaves WRT project creation.

[Copilot]

Pull request overview

Expands the Dependency-Track access control documentation to clarify the relationship between permissions and Portfolio Access Control (PAC), and relocates authentication-flow material into the relevant authentication guides.

Changes:

• Restructures docs/concepts/access-control.md: clarifies that permissions and ACLs are orthogonal, adds a separating-teams pattern with a Mermaid diagram, and documents ACL inheritance, the PORTFOLIO_ACCESS_CONTROL_BYPASS bypass, and project-creation behavior under PAC.
• Moves the LDAP and OIDC authentication-flow descriptions out of the access-control concept page and into their respective administration guides.
• Adds the PAC abbreviation and minor copy edits in the OIDC guide.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
docs/concepts/access-control.md	Major expansion of PAC documentation; removes auth flow sections; rewrites effective-permissions note.
docs/guides/administration/configuring-ldap.md	Adds "Authentication flow" section relocated from the concepts page.
docs/guides/administration/configuring-oidc.md	Adds "Authentication flow" section and a small wording tweak in the frontend note.
docs/includes/abbreviations.md	Adds PAC abbreviation expansion.

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