[doc] Update GettingStarted.tid on tw.com

[twMat]

The ultimate goal for this tiddler probably requires a fully new approach - but meanwhile, this PR can be seen as a small improvement of what we have.

If the PR is not liked, please point out what is not good.

Note: I would like for this tiddler to also inform that native TW does have a built-in saver and that the other savers are really optional and for convenience (which is not clear in the current GettingStarted!) but I am not sure how to phrase this. This particular aspect is also related to #6810