Add instructions on finding the ID of an existing extension

[Rob--W]

This PR adds instructions on finding the ID of an existing extension.

Note that the instructions include "install" as part of the workflow. This is acceptable for workflows where one wants to update their own extension. I can imagine scenarios where one does not want or cannot install the previous version of the extension. In that case, they could visit https://robwu.nl/crxviewer/ , specify the URL of the extension (or select the xpi file in the file chooser), then click on manifest.json in the sidebar, then click on the Show analysis button to reveal the Extension ID field.

It also fixes a few minor issues:

• doubly listed contributor
• incorrectly formatted link due to use of ] instead of ) to close [...](...)
• Remove However, from a sentence that did not describe an opposing statement.

[rebloor]

@Rob--W left some suggestions for your consideration

[rebloor]

Suggested change

	To find the ID for an extension that has already been published, install your existing extension from AMO, visit `about:debugging` and look at the `Extension ID` field of your extension.
	To find a published extension's ID, install the extension from AMO, visit `about:debugging`, and check the **Extension ID** field.

[rebloor]

Suggested change

	This ID is shown as `Extension ID` at `about:debugging` after the add-on is installed. This ID is the same for all users of the add-on.
	After the add-on is installed, the ID is shown in the `about:debugging` **Extension ID** field. This ID is the same for all users of the add-on.

[rebloor]

Suggested change

	And if you update your extension to use Manifest Version 3, [the add-on ID must be set](#how-do-i-set-an-add-on-id).
	And, if you update your extension to use Manifest Version 3, [the add-on ID must be set](#how-do-i-set-an-add-on-id).

[wagnerand-moz]

There must be a better way to view the ID on AMO, and not naving to reside to a personal website or installing the add-on. Can we start exposing this on the AMO listing page?

Maybe we can also have a dedicated section on this page, rather than repeating the same information several times. That section could also include to open the manifest file and inspect browser_specific_settings, if you have the XPI file, or only have that, e.g. for self-hosted add-ons.

[uncenter]

There must be a better way to view the ID on AMO, and not naving to reside to a personal website or installing the add-on. Can we start exposing this on the AMO listing page?

I'm wondering if it is the same as this UUID field? It's labeled as UUID but the ? tooltip says "The UUID of your add-on is specified in its install manifest and uniquely identifies it. You cannot change your UUID once it has been submitted." Seems to be conflating the UUID format/terminology with the manifest ID so I'm not sure.

[wagnerand-moz]

@uncenter Yes, in this specific case UUID and Add-on ID are the same. Sometimes we also call it GUID. It's confusing, I know. To add even more to that, Firefox also has an install-specific UUID that is exposed on about:debugging. We should probably rename that field in devhub to "Add-on ID", as that is the term we are using publicly.

@Rob--W could you file an issue on https://github.com/mozilla/addons/ to ask that we show the add-on ID in the "More information" section on the listing page?

@uncenter raises a good point. Developers generally know how to look up their ID or at least can access it through that devhub field. This documentation might be used by users or developers who don't own the particular add-on they want to look up the ID for, for example to find the listing page, or to report it to Mozilla.
If we want to advise non-owners to install an add-on just to get its ID, we might want to add a notice that this should be done in a separate profile (thinking about the reporting case). We probably don't have to guide them through how to do that, I assume there is a SUMO page we can link to.

[Rob--W]

@Rob--W could you file an issue on https://github.com/mozilla/addons/ to ask that we show the add-on ID in the "More information" section on the listing page?

Done: mozilla/addons#16011

I will wait a little bit with updating this PR, say 1-2 weeks, to see if the feature request can be addressed quickly. If so, then the instruction on getting the ID would be much better than "install the extension". I don't really want to recommend installing the extension to know the extension ID if possible. While it may be acceptable for self-developed extensions, it is a bad practice for arbitrary third-party extensions.