Umbraco 15.4 release documentation updates

[AndyButland]

Description

Documentation updates related to the 15.4 release:

• Added details of allowed content type filters - see https://github.com/umbraco/Umbraco-CMS/pull/18380
• Added details of multiple segments per document/culture combination - see https://github.com/umbraco/Umbraco-CMS/pull/18603

Type of suggestion

• [ ] Typo/grammar fix
• [ ] Updated outdated content
• [ ] New content
• [X] Updates related to a new version
• [ ] Other

Product & version (if relevant)

Umbraco 15.4

Deadline (if relevant)

Will keep in draft as we will likely add to this.