docs: improve action, attach documentation

[hyunbinseo]

Describe the problem

As a long time Svelte user, this was my frustration:

1. Forgot about what the term attach
2. Remembered previously used Action
3. No mention of attach in that documentation
4. Moved to the adjacent svelte/attachments docs
5. Does not explain the term and starts with createAttachmentKey
6. Learns that there is a separate docs for it, linked at the end of the page

Describe the proposed solution

This PR introduces attachments, which are essentially a more flexible and modern version of actions. #15000

1. Advise attachments in the svelte/action docs
2. Reorder svelte/attachments docs and introduce Attachment in the beginning

Importance

nice to have