[repo-assist] Support top-level (seealso) XML doc tags in API docs

[github-actions]

🤖 This PR was created by Repo Assist, an automated AI assistant, in response to a /repo-assist command from @dsyme on issue #1256.

Closes #1256

Root cause

Per the [xmldoc recommended tags]((learn.microsoft.com/redacted), <seealso cref="..."/> is meant to be a top-level doc-comment section (distinct from the inline <see cref="..."/> tag). FSharp.Formatting's XML doc reader (XmlDocReader.fs) only handled <seealso> as an inline tag (identically to <see>), so top-level <seealso> elements were silently dropped and never rendered.

Fix

• Added a SeeAlso: (string * string option * ApiDocHtml) list member to ApiDocComment, modeled on the existing Exceptions field.
• XmlDocReader.readXmlCommentAsHtmlAux now collects direct-child <seealso> elements (resolving cref via the existing cross-reference resolver, same as <exception cref="...">), and excludes them from the generic "unrecognised tag" raw-data bucket.
• combineComments now merges SeeAlso lists when doc comments are combined.
• Both GenerateHtml.fs and GenerateMarkdown.fs render a "See also" section (as a bullet/list of links) for members and entities that have one or more top-level <seealso> tags, following the existing rendering style used for Notes/Examples.

Inline <see cref="..."/> behavior (and the pre-existing inline handling of <seealso> embedded in running text) is unchanged.

Trade-offs / notes

• Only direct child <seealso> elements of the member/type doc comment are treated as a top-level section; a <seealso> nested inside <para>/<summary> etc. still falls back to inline rendering (same as <see>), which matches how <exception> is already handled.
• No new dependencies added; change is scoped to FSharp.Formatting.ApiDocs.

Test Status

• ✅ Build succeeded (dotnet build src/FSharp.Formatting.ApiDocs/... -c Release, dotnet build tests/FSharp.ApiDocs.Tests/... -c Release)
• ✅ Added 2 new tests (ApiDocs Markdown generates See also section for top-level seealso tags, ApiDocs HTML generates See also section for top-level seealso tags) exercising a new SeeAlsoExamples module added to the FsLib2 test fixture.
• ✅ Full FSharp.ApiDocs.Tests suite: 90 passed, 0 failed, 4 skipped (pre-existing skips, unrelated to this change)
• ✅ dotnet fantomas src tests --check — no formatting issues

As always, a human maintainer should review before merging. Thanks for reporting, @Tarmil!

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