Add 4.5.1 release notes

[javimed]

Description

This PR adds a document with the release notes for version 4.5.1

Checks

Docs building

• Compiles without warnings.

Code formatting and web optimization

• Uses three spaces indentation.
• Adds or updates meta descriptions accordingly.
• Updates the redirects.js script if necessary (check this guide).

Writing style

• Uses present tense, active voice, and semi-formal registry.
• Uses short, simple sentences.
• Uses bold for user interface elements, italics for key terms or emphasis, and code font for Bash commands, file names, REST paths, and code.

[s-ocando]

I've reviewed your PR, and it looks good. Nevertheless, I've left a few suggestions below. Please review them.

• Add the 4.5.1 release notes page to the redirects.js file under newUrls['4.5'].
• Consider removing PR #2224 from the packages section since it's included in the Agent section. As it's the highlight of the release, consider placing it as the first item in the Agents section.
• Consider including a link to the Splunk app change log.