Release Notes of docfx vNext version

[filzrev]

Currently it seems there is no breaking changes.
So next version is expected to be v2.78.5.

And at least following PR need to be merged before next version released.

• https://github.com/dotnet/docfx/pull/10966

Release Note that manually edited.

## What's Changed

## 🌟 New Features
- 🧩 Updated to Roslyn 5.0.0 and added .NET 10 support. (#10859)
- 🧩 Add support for C# 14 syntax with following limitation. (#10943)
  -  Extension members are not shown on generated API document.

## 🐞 Bug Fixes
* ✅ Fix invalid link is generated when using SourceGenerator (#10850)
* ✅ Fix layout broken on specific documentation comment tags combination (#10553/#10965)
* ✅ Fix exception logging behaviors (#10945/#10946)
* ✅ Fix navigation layout issue on mobile display size (#10835)
* ✅ Fix failed to find xref issue on Alpine docker image (#10900)
* ✅ Fix derivedClasses is not shown on API page of interface (#2531)
* ✅ Fix old cache content shown on `docfx serve` issue by setting `cache-control` explicitly (#10891)

**Full Changelog**: https://github.com/dotnet/docfx/compare/v2.78.4...v2.78.5

How to get draft ReleaseNote content from repository.

It can get automatically generated draft content by following command (It's not affecting repository)

$tag_name          = 'v2.78.5' # This tag name may be not exists.
$target_commitish  = 'main'
$previout_tag_name = gh release list --limit 1 --json tagName --jq '.[0].tagName'

$json = gh api repos/dotnet/docfx/releases/generate-notes `
            -f tag_name=$tag_name                          `
            -f target_commitish=$target_commitish          `
            -f previous_tag_name=$previout_tag_name

$result = $json | ConvertFrom-Json


Write-Host "## ${$result.name}" # Title(Tag Name)
Write-Host $result.body              # Draft Release Notes (Markdown)