Fixed #15727 -- Added Content Security Policy (CSP) support.

This initial work adds a pair of settings to configure specific CSP
directives for enforcing or reporting policy violations, a new
`django.middleware.csp.ContentSecurityPolicyMiddleware` to apply the
appropriate headers to responses, and a context processor to support CSP
nonces in templates for safely inlining assets.

Relevant documentation has been added for the 6.0 release notes,
security overview, a new how-to page, and a dedicated reference section.

Thanks to the multiple reviewers for their precise and valuable feedback.

Co-authored-by: Natalia <124304+nessita@users.noreply.github.com>

docs/topics/security.txt

@@ -286,6 +286,61 @@ User-uploaded content
 
 .. _same-origin policy: https://en.wikipedia.org/wiki/Same-origin_policy
 
+.. _security-csp:
+
+Content Security Policy
+=======================
+
+.. versionadded:: 6.0
+
+Content Security Policy (CSP) is a browser security mechanism that helps
+protect web applications against attacks such as cross-site scripting (XSS) and
+other content injection attacks.
+
+CSP allows web applications to define which sources of content are trusted,
+instructing the browser to load, execute, or render resources only from those
+sources. This effectively creates an allowlist of content origins, reducing the
+risk of malicious code execution.
+
+Key benefits of enabling CSP include:
+
+1. Mitigating XSS attacks by blocking inline scripts and restricting external
+   script loading.
+2. Controlling which external resources (e.g., images, fonts, stylesheets) can
+   be loaded.
+3. Preventing unwanted framing of your site to protect against clickjacking.
+4. Reporting violations to a specified endpoint, enabling monitoring and
+   debugging.
+
+For configuration instructions, see the :ref:`Using CSP <csp-config>`
+documentation, and refer to the :ref:`CSP overview <csp-overview>` for details
+on directives and settings.
+
+Limitations and considerations
+------------------------------
+
+While CSP is a powerful security mechanism, it's important to understand its
+limitations and implications, particularly when used in Django:
+
+* Policy exclusion risks: Avoid excluding specific paths or responses from
+  CSP protection. Due to the browser’s same-origin policy, a vulnerability on
+  an unprotected page (e.g., one allowing arbitrary script injection) may be
+  leveraged to attack protected pages. Excluding *any* route can significantly
+  weaken the site's overall CSP protection.
+
+* Performance overhead: Although typically negligible, CSP adds some processing
+  overhead. Nonce generation involves secure randomness for each applicable
+  request. For high-traffic applications or resource-constrained environments,
+  measure the performance impact accordingly.
+
+* Browser support: While CSP Levels 1 and 2 are widely supported, newer
+  directives (CSP Level 3+) or complex policy behaviors may vary across
+  browsers. Test your policy across the environments you intend to support.
+
+Despite these limitations, CSP remains an important and recommended security
+layer for web applications. Understanding its constraints will help you design
+a more effective and reliable deployment.
+
 .. _additional-security-topics:
 
 Additional security topics