[Java.Interop.Tools.JavaSource] Add {@ docRoot} support

[pjcollins]

Context: https://docs.microsoft.com/en-us/dotnet/api/android.app.backup.backupagent.onrestore?view=xamarin-android-sdk-12#Android_App_Backup_BackupAgent_OnRestore_Android_App_Backup_BackupDataInput_System_Int64_Android_OS_ParcelFileDescriptor_
Context: https://docs.microsoft.com/en-us/dotnet/api/android.animation.animatorlisteneradapter.onanimationend?view=xamarin-android-sdk-12#definition
Partially fixes: #907

Support for {@ docRoot} has been added, and an additional
parser for <a href/> elements has been added to convert
these to <see/> elements.

[pjcollins]

This has generated the following docs diff:
https://gist.github.com/pjcollins/14b246c8c2a97336184d3bb3328c619a

There are still some issues to sort out.

[pjcollins]

Latest docs diff for these changes (as compared to a recent partial update for API 32) - https://gist.github.com/pjcollins/355c3acb20c509c00cfffebe30744082

[jonpryor]

<see href="…"/> is a valid construct? I wonder when that happened…

[jonpryor]

Looks like it happened in 2017. Now I feel old. mono/api-doc-tools@fb07047

[jonpryor]

WIP commit message:

Partially fixes: https://github.com/xamarin/java.interop/issues/907

Context: https://docs.microsoft.com/en-us/dotnet/api/android.app.backup.backupagent.onrestore?view=xamarin-android-sdk-12#Android_App_Backup_BackupAgent_OnRestore_Android_App_Backup_BackupDataInput_System_Int64_Android_OS_ParcelFileDescriptor_
Context: https://docs.microsoft.com/en-us/dotnet/api/android.animation.animatorlisteneradapter.onanimationend?view=xamarin-android-sdk-12#definition

Original support for the [`{@docRoot}` inline Javadoc tag][0] in
commit 7574f166 was a "TODO"; `{@docRoot}` would expand to
`[TODO: @docRoot]`, resulting in documentation containing such
monstrosities as:

> The value of the
> <a href="[TODO: @docRoot]guide/topics/manifest/manifest-element.html#vcode">android:versionCode</a>
> manifest attribute…

Add support for the `{@docRoot}` inline tag, along with grammar
support for `<a href="…">…</a>` elements.  This allows:

	<a href="{@docRoot}/example/path">description</a>

to be converted into:

	<see href="http://example.com/example/path">description</see>

The value of `{@docRoot}` is expanded into
`XmldocSettings.DocRootValue`, which comes from the
`/api/javadoc-metadata/link/@docroot` element in "Javadoc XML"
created by `java-source-utils.jar` (7574f166), via the new
`java-source-utils.jar --doc-root-url URL` option:

	java -jar java-source-utils.jar \
	  --doc-root-url https://developer.android.com/ \
	  --doc-url-prefix https://developer.android.com/reference \
	  --doc-url-style developer.android.com/reference@2020-Nov \
	  …

TODO: Update the [`@(JavaSourceJar)` Build action][1] so that e.g.
`%(JavaSourceJar.DocRootUrl)` item metadata will be used as the
`java-source-utils.jar --doc-root-url` value.

[0]: https://docs.oracle.com/javase/7/docs/technotes/tools/windows/javadoc.html#docRoot
[1]: https://docs.microsoft.com/en-us/xamarin/android/deploy-test/building-apps/build-items#javasourcejar