DOCS Review 4.11.0 changelog notes

[GuySartorelli]

• Add missing notes from the beta and rc changelogs
• Add notes for changes that developers are likely to want to know about
• Move php notes under the features heading (consistent with its position in the 4.10.0 changelog)
• Formatting and phrasing changes
• Add documentation to the "Customise site reports" section that was added to the reports module readme but really belongs here.

This PR does not add the actual changelog (i.e. references to individual commits), which will be generated closer to release time.

Parent issue

• https://github.com/silverstripeltd/product-issues/issues/537

[maxime-rainville]

Generally happy