Add explicit mention to the required "application/x-xpinstall" response content type for self distribution xpi files from web urls

[rpl]

InstallTrigger used to ignore the response content type, but setting the expected "application/x-xpinstall" response content type to successfully trigger the addon install flow through a navigation to an xpi file url is actually mandatory.

In many case that is already the default config, but it may not be always be the case and so developers self-distributing their addons and migrating away from the deprecated InstallTrigger may hit an issue with successfully triggering the install flow and it may not be immediately clear what is missing in their webserver setup (e.g. Bug 1784440).

We should add an explicit mention to this requirement in the related section:

extension-workshop/src/content/documentation/publish/self-distribution.md

Line 66 in 828887d

### Web download