[rum_salesforce_lwc] Document Session Replay setup for Experience Cloud

[BeltranBulbarellaDD]

What does this PR do?

Documents how to enable Session Replay for Salesforce Experience Cloud sites through Head Markup.

This update:

• Adds setup instructions for Salesforce Static Resources, asynchronous CDN loading, and synchronous CDN loading.
• Documents the required CSP configuration and lazy-loaded chunk resources.
• Clarifies that Session Replay is supported only through Head Markup because Lightning Web Security prevents it from running inside Lightning components.
• Updates the feature support matrix and configuration examples.
• Adds the 1.1.0 changelog entry.

It's exactly the same Readme as in Browser SDK repo https://github.com/DataDog/browser-sdk/blob/main/packages/browser-rum/src/salesforce/README.md
added here: DataDog/browser-sdk#5036

Motivation

Users need a supported setup path for collecting Session Replay data from Experience Cloud sites. The existing documentation only covered RUM initialization and did not explain the Head Markup requirement, CSP allowlisting, or the additional resources needed by Session Replay.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo
• If this PR includes a log pipeline, please add a description describing the remappers and processors.

Additional Notes

Documentation-only change; no integration runtime code is modified.

[dd-octo-sts]

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