fix: docfx download command behaviors

[filzrev]

This PR intended to fix problems commented at #9659 (comment)

Background
When running docfx download with following command.

docfx download xrefmap.zip --xref https://github.com/dotnet/docfx/raw/main/.xrefmap.json

Xrefmap is downloaded as xrefmap.zip.
However, the contents of xrefmap.yml are modified as follows.

1. baseUrl (https://learn.microsoft.com/dotnet/api/) is updated to https://github.com/dotnet/docfx/raw/main/.
2. href with relative URLs are rewritten to absolute URL that based on https://github.com/dotnet/docfx/raw/main/.

This PR fix above behaviors by removing relating code.
And this changes. docfx download command simply execute following tasks.

• Download .json or .yml file from specified URL.
• Load downloaded content as XRefMap model. And serialize data to xrefmap.yml.
• Sort XRefMap's references by UID to optimize performance.
• Save zip archive as specified file name. (xrefmap.zip)

---

What's included in this PR

• Remove baseUrl/href update related logics that are used when docfx download command.
• Remove unused code. (Relating CreateMinor method)
• Add documentation comment for XRefMap type.
• Add warning messages when specify zip archive to download URL. (Currently it seems silently ignored. See: docfx download throws exception #2805)
• Sort references by UID to optimize performance (for not sorted xrefmap),
  and to support not properly sorted xrefmap. (sorted:true but not properly sorted) (See: Some Unity uids could not be resolved by xrefmap NormandErwan/UnityXrefMaps#4)

Test
I've tested docfx download command with following xrefmaps.
And It works as expected.

• https://github.com/dotnet/docfx/raw/main/.xrefmap.json
• https://normanderwan.github.io/UnityXrefMaps/xrefmap.yml

Additional notes
This PR contains some docfx download command behavior changes.
It might be better to list changelog as BREAKIN CHANGES.

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 74.40%. Comparing base (fe673ec) to head (6e665d3).
Report is 36 commits behind head on main.

Files	Patch %	Lines
src/Docfx.Build/XRefMaps/XRefArchiveBuilder.cs	69.23%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9721      +/-   ##
==========================================
+ Coverage   74.31%   74.40%   +0.08%     
==========================================
  Files         536      536              
  Lines       23189    23192       +3     
  Branches     4056     4027      -29     
==========================================
+ Hits        17234    17255      +21     
+ Misses       4853     4842      -11     
+ Partials     1102     1095       -7     

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[yufeih]

Looks good!