[Repo Assist] Split XmlDocReader out of SymbolReader.fs (#1073)

* Split XmlDocReader out of SymbolReader.fs

Extract XML and Markdown documentation comment reading helpers
(~440 lines) into a new XmlDocReader.fs file. These functions form
a coherent unit (XML → ApiDocComment conversion) that does not
depend on ReadingContext; they only need CrossReferenceResolver
and the ApiDoc* types.

- New: src/FSharp.Formatting.ApiDocs/XmlDocReader.fs
  - removeSpaces
  - readMarkdownCommentAsHtml
  - findCommand
  - readXmlElementAsSingleSummary / readXmlElementAsHtml
  - readXmlCommentAsHtmlAux / readXmlCommentAsHtml / readNamespaceDocs
  - combineHtml / combineHtmlOptions / combineComments / combineNamespaceDocs
- Modified: SymbolReader.fs shrinks from 1703 → 1263 lines

Part of #1022.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* ci: trigger checks

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>

Author: 3 people

src/FSharp.Formatting.ApiDocs/FSharp.Formatting.ApiDocs.fsproj

@@ -13,6 +13,7 @@
     <Compile Include="ApiDocTypes.fs" />
     <Compile Include="CrossReferenceResolver.fs" />
     <Compile Include="TypeFormatter.fs" />
+    <Compile Include="XmlDocReader.fs" />
     <Compile Include="SymbolReader.fs" />
     <Compile Include="GenerateModel.fs" />
     <Compile Include="Categorise.fs" />