Update presence design document

[chacha912]

What this PR does / why we need it:

Update design document for presence.
It contains information about the changes made in the following pull request.

• Move Presence to Document:
  • Move Presence to Document #582
  • Move Presence from Client to Document yorkie-js-sdk#574
  • Replace selection with presence yorkie-js-sdk#578

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

While working on the document, I think it is better to make the following changes. (I will address these changes through a separate pull request):

• Change "documents" to "document" in DocEvent (since the WatchDocument API has been updated to operate on a single document: Add RemoveDocument API yorkie-js-sdk#480).
• Rename doc.getPresences() to doc.getUsers() (as the returned value aligns more closely with user information, making this name easier to comprehend).

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #609 (53ef2a0) into main (65c8164) will decrease coverage by 0.96%.
Report is 4 commits behind head on main.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #609      +/-   ##
==========================================
- Coverage   51.19%   50.24%   -0.96%     
==========================================
  Files          67       69       +2     
  Lines        7053     7314     +261     
==========================================
+ Hits         3611     3675      +64     
- Misses       2960     3147     +187     
- Partials      482      492      +10     

Files Changed	Coverage Δ
server/backend/backend.go	0.00% <0.00%> (ø)

... and 11 files with indirect coverage changes

[hackerwins]

LGTM. I left a minor comment.