chore(docs): add documentation structure for MetaChain

[fuxingloh]

What this PR does / why we need it:

As per the title. Merely cosmetic change to move documentation structure into place.

Which issue(s) does this PR fixes?:

Fixes part of #8

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[codecov]

Codecov Report

Merging #90 (0feb4a8) into main (6586bc9) will increase coverage by 77.50%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            main      #90       +/-   ##
==========================================
+ Coverage   8.21%   85.71%   +77.50%     
==========================================
  Files         16        6       -10     
  Lines       1814       98     -1716     
  Branches      14       14               
==========================================
- Hits         149       84       -65     
+ Misses      1664       13     -1651     
  Partials       1        1               

Impacted Files	Coverage Δ
meta/meta-node/src/rpc.rs
meta/meta-consensus/src/lib.rs
meta/meta-node/src/service.rs
meta/meta-runtime/src/tests.rs
meta/meta-runtime/src/lib.rs
meta/meta-node/src/cli.rs
meta/meta-node/src/main.rs
meta/meta-node/src/chain_spec.rs
meta/meta-runtime/src/mock.rs
meta/meta-node/src/command.rs

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[github-actions]

Docker image for defich/metachain is ready!

Built with commit a98c50e

• docker pull ghcr.io/defich/metachain:a98c50e19b84243cee2a80316f416254db1b9fd1
• docker pull ghcr.io/defich/metachain:pr-90