Need to add sitemap.xml support

[Aaronontheweb]

DocFX Version Used: any

Template used: default

DocFx should really support the ability to produce an XML sitemap; we recently migrated to DocFx on getakka.net and Google is picking up none of our URLs and links.

[vicancy]

According to https://www.sitemaps.org/protocol.html, This URL for loc must begin with the protocol (such as http). Need to provide the host name when docfx build for generating sitemap.xml

[vicancy]

This feature will release in v2.25.
To enable this feature, in "build" section of "docfx.json", at least specify the following:

"sitemap": {
  "baseUrl": "https://yourhostname.com/"
}

To customize more properties, follow https://github.com/dotnet/docfx/blob/dev/Documentation/tutorial/docfx.exe_user_manual.md#324-sitemapoptions

[Aaronontheweb]

@vicancy I never followed up on this, but thank you for adding this feature! I really appreciate it.