Add tree document

[MoonGyu1]

What this PR does / why we need it:

• Write a document about concurrent tree editing to maintain a specification
• The core table of contents is as follows:

1. XML-like Tree

• Edit/Style Operations

2. Implementation of Edit Operation

• Tree Coordinate System
• Tree.Edit Logic

3. How to Guarantee Eventual Consistency

• Coverage
• How does it work

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

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 #608 (2691c5f) into main (21b3afc) will decrease coverage by 1.49%.
Report is 11 commits behind head on main.
The diff coverage is 48.81%.

@@            Coverage Diff             @@
##             main     #608      +/-   ##
==========================================
- Coverage   50.89%   49.41%   -1.49%     
==========================================
  Files          69       69              
  Lines        7167     9914    +2747     
==========================================
+ Hits         3648     4899    +1251     
- Misses       3035     4501    +1466     
- Partials      484      514      +30     

Files Changed	Coverage Δ
api/converter/to_pb.go	52.63% <0.00%> (-2.62%)	⬇️
pkg/document/internal_document.go	15.34% <0.00%> (+1.66%)	⬆️
server/rpc/admin_server.go	50.00% <0.00%> (-3.98%)	⬇️
client/client.go	13.58% <8.33%> (+0.09%)	⬆️
api/converter/to_bytes.go	57.37% <15.78%> (-5.57%)	⬇️
api/converter/from_pb.go	42.12% <20.45%> (-6.88%)	⬇️
pkg/document/crdt/tree.go	52.49% <53.95%> (-2.89%)	⬇️
pkg/index/tree.go	56.30% <69.35%> (+7.45%)	⬆️
server/rpc/yorkie_server.go	50.52% <92.85%> (-6.31%)	⬇️

... and 60 files with indirect coverage changes

[hackerwins]

@MoonGyu1 Could you merge the main branch here?

[hackerwins]

I left a comment on the term.

[hackerwins]

I left a minor comment about the title.

[hackerwins]

I left one more minor comment.

[krapie]

It seems like the tree document link is missing in README.md in the design document.
I will update this on #630