Oxygen add-on for XSpec v3.2.0

[galtm]

In xspec/xspec#2075 (comment), @cmarchand suggests publishing oxygen-addon.xml at xspec.io. This pull request does that for the existing v3.2.0 add-on for which Oxygen users would point Oxygen to the URL https://github.com/xspec/xspec/raw/master/oxygen-addon.xml (instructions). The purpose of this PR is to make sure this new publishing location works and that we're happy with it, before asking users to change their Oxygen setting.

After this pull request is merged, Oxygen should be able to use the following URL to find and install the add-on: https://xspec.io/editors/oxygen/oxygen-addon.xml

I am open to suggestions about what the URL should be.

In this pull request,

• The oxygen-addon.xml file is from the top level of the xspec/xspec repo, and I made two changes: I flattened the XInclude pointing to LICENSE because a LICENSE file in this repo might look as if it's the license for this repo rather than the license for the XSpec add-on code. Also, I redirected the XInclude pointing to latest.xhtml to adapt to the shallower directory hierarchy in this repo.
• The latest.xhtml file is directly from the xspec/xspec repo, but I used a shallower directory hierarchy because it seemed simpler. The original file is at https://github.com/xspec/xspec/blob/0bef40c0dbed817717ff1b1a8043f367b368d33b/editors/oxygen/add-on/description/latest.xhtml

Testing done

I was able to install the add-on in Oxygen 24.1 and Oxygen 27.0, after running hugo locally and giving Oxygen this URL:
http://localhost:1313/editors/oxygen/oxygen-addon.xml

In one case, I had the v3.1.3 add-on previously installed, and Oxygen removed the v3.1.3 add-on when unpacking the new one. It also recognized the new one as an update of the old one.

[galtm]

I'm going to merge this pull request. The stakes should be low because the URL https://xspec.io/editors/oxygen/oxygen-addon.xml hasn't been announced in the XSpec wiki or in any publicity channels.

[galtm]

I was able to install the add-on using Oxygen (26.1) and the URL https://xspec.io/editors/oxygen/oxygen-addon.xml after removing from my Oxygen preferences the other URLs that I've used in the past for installing the XSpec Oxygen add-on. I can see the com.oxygenxml\extensions\v26.1\frameworks\io.xspec.xspec\xspec-fee92fd453ac9e55e3c889df168ebc918fdc8485 folder in my local Oxygen data area, and I see the new src\reporter\test-report-colors-blackwhite.css file that is new in v3.2.0.