Resolve "Move documentation images away from the `main` branch"

This MR solves the issue where we couldn't change any README images (without breaking earlier version READMEs) because we referred to them using only the main branch. This is how the links changed:

- https://gitlab.com/gitlab-org/gitlab-vscode-extension/raw/main/src/assets/_diff-comments.gif
+ https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/raw/d0589878829338b64657e592f3451f1dace41cdf/docs/assets/diff-comments.gif

1. Instead of referring to images using main branch, we use the commit SHA. This means that in the future, we can delete images that are no longer used without breaking README for earlier versions of the extension (but we'll still break versions before this MR).
2. The readme images were copied from src folder to docs folder because they are unrelated to the extension source.
3. The links now contain the /-/ delimiter.
4. The _ prefix got removed because we don't have to distinguish runtime and documentation assets.

This MR only copies the images. We can remove them from src/assets in a few months when we don't mind breaking a few months old README.

See the new README.md.

I've chosen this solution (permalinks and docs/assets folder) over putting images into a separate branch because this solution allows contributors to add the images themselves. Having a separate protected branch would mean that maintainers need to upload the images for them.

Related to #432 (closed)