Add comprehensive What's New documentation for F# 7

[Copilot]

Summary

Replaces placeholder blog link with full documentation for F# 7 features following F# 9 template structure.

Fixes #50481

Changes

The F# 7 documentation now includes:

• Static abstract members in interfaces - Runtime feature enabling generic math via interface constraints with static methods, demonstrated with IAddition<'T> and INumber<'T> examples
• Simplified SRTP syntax - Major ergonomic improvement allowing 'T instead of ^T for type parameters, natural member call syntax ('T.Zero vs explicit constraint invocation), and grouped constraint declarations
• Required properties checking - C# 11 interop with compile-time enforcement of required member initialization
• Init scope enforcement - Breaking change tightening init-only property mutation to initialization context only
• Reference assemblies support - Compiler capability via --refout/--refonly flags or MSBuild properties
• Platform and performance improvements - ARM64 support, trimmability flags, parallel type-checking (experimental), N-d arrays up to rank 32

Code examples demonstrate practical usage patterns rather than abstract syntax. Documentation follows established F# 9 style with inline examples, sentence case headings, and active voice.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Missing documentation for F# 7</issue_title>
<issue_description>
This is really not the page that has the issue, rather the parent section "What's new".

There is no documentation for what is new in F# 7, beyond this blog post, that only savy developers will manage to find out.

---

Document Details

⚠ Do not edit this section. It is required for learn.microsoft.com ➟ GitHub issue linking.

• ID: e2dea170-a8f4-90f6-e578-e406fb43fef1
• Version Independent ID: 885f9f09-d2ff-112c-6dda-9a31bccc0e8a
• Content: What's new in F# 6 - F# Guide
• Content Source: docs/fsharp/whats-new/fsharp-6.md
• Product: dotnet-fsharp
• GitHub Login: @cartermp
• Microsoft Alias: dotnetcontent

---

Associated WorkItem - 498946</issue_description>

<agent_instructions>Find docs pages for WhatsNew F#7.
There is an attached link to a blog post. Read it and follow your instructions for properly copying content with code samples from blogs to docs.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@BillWagner ping @KathleenDollard @dsyme

Who would be best to adapt the blog post for docs?</comment_new>
<comment_new>@BillWagner
The content for "What's new" in F# 7 and F# 8 are only pointers to the blog post that announced those features.

Instead, both should follow the example of F# 9 and have that blog content replicated in docs, with the appropriate docs styles.</comment_new>

• Fixes Missing documentation for F# 7 #33166

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[abonie]

Fix the links, correct link to the C# feature is this: https://learn.microsoft.com/en-us/dotnet/csharp/advanced-topics/interface-implementation/static-virtual-interface-members and correct link to the generic-math section is https://learn.microsoft.com/en-us/dotnet/csharp/advanced-topics/interface-implementation/static-virtual-interface-members#generic-math

Also make it clear that the link leads to documentation of the corresponding feature on C#'s side.