Add Macro documentation

[loliGothicK]

Add a macro entry to the document.

[loliGothicK]

1. make new markdown files.

docs/
├ ...
├ mkdocs.yml
└ docs
    ├ ...
    ├ maybe/
    │    ├ ...
+   │    └ macros.md
    └ result/
         ├ ...
+        └ macros.md

2. write documents

   • PANIC
   • MITAMA_TRY

3. add pages to page tree in docs/mkdocs.yml

# Page tree
nav:
  - Home: README.md
  - Maybe Mudule:
    - Maybe 101: maybe/intro.md
    - API Reference: maybe/APIs.md
    - Special Members: maybe/special_members.md
    - Comparisons: maybe/comparisons.md
    - Meta Functions: maybe/metafunctions.md
+   - Macros:  maybe/macros.md
  - Result Module:
    - Result 101: result/intro.md
    - Special Members: result/special_members.md
    - API Reference: result/APIs.md
    - Meta Functions: result/metafunctions.md
    - Comparisons: result/comparisons.md
+   - Macros:  result/macros.md
    - CHANGELOG: result/CHANGELOG.md