Holistic instructions for uploading extensions to Open VSX

# Why

Recently I've been trying to help an extension author to bring their extension to Open VSX. This is something they don't use themselves, so in an attempt to convince them, I was looking for some instructions that guide them through the entire process. However, I have discovered that instructions aren't complete and require you to click-through to several pages.

By making the process as accessible, easy and complete as possible, it becomes easier for both extension authors and extension users to implement

# How

I hope we can improve the wiki by creating a holistic guide, i.e. complete from beginning to end, on how to get your VS Code extension automatically deploying to both the VS Marketplace and Open VSX, using GitHub Actions and the [HaaLeo/publish-vscode-extension](https://github.com/HaaLeo/publish-vscode-extension) action. instructions would include (but not limited to, in case I forget something):

* Signing any necessary documents
* Installing necessary tools, like ovsx
* Copy-paste ready CI pipelines that are kept up-to-date with Dependabot and [rhysd/actionlint](https://github.com/rhysd/actionlint/). The current one is pretty outdated with set-output and outdated actions. See also PR https://github.com/EclipseFdn/publish-extensions/pull/946)
* Instructions for acquiring any necessary tokens, and saving them in the GitHub secrets
* Optionally, verifying ownership

Screenshots are also a very powerful and helpful tool, and it would be best if these are included.

I would suggest requiring at most a single click-through when referencing to articles. For example, it would make sense to guide to GitHub documentation on how to set secrets, but linking to another step-by-step guide that is incomplete and includes links to other documentation pages would quickly unravel for the end-user.

Preferably when creating this guide we try this from a clean slate, perhaps with only npm set up. The scope of the tutorial would, in my opinion, be limited to Eclipse repositories, Open VSX and GitHub instructions, and exclude scope such as setting up npm, since an extension author must already be familiar with npm, but not necessarily GitHub secrets.

# Who should do this

I might pick up this issue myself, but I am no extension author. Hopefully a maintainer, or anyone with a day off will be interested in picking this up. This issue is probably worthy of a "good-first-issue" label.

Start by reviewing the existing wiki instructions, the HaaLeo/publish-vscode-extension action, and linked PR #946. Verify the complete Eclipse, Open VSX, VS Marketplace, GitHub Actions, token, signing, and ownership flow from a clean setup. Done means one maintained guide with copy-paste-ready pipelines, screenshots, and no more than one click-through for referenced documentation.

Written by the indexing model from the issue text.