Add document

Generate document by vimdoc
Add a pre-commit-hook to call vimdoc automatically after git commit

.pre-commit-config.yaml

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+---
+repos:
+  - repo: https://github.com/google/vimdoc
+    rev: 73bc1efaacb55a59d5206d0ac50e5432ea0192c1
+    hooks:
+      - id: vimdoc

doc/.gitignore

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-*
-!/.gitignore

doc/requirements.txt

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+*requirements.txt*	the Requirements File Format syntax support for Vim
+raimon                                                          *requirements*
+
+==============================================================================
+CONTENTS                                               *requirements-contents*
+  1. Introduction.........................................|requirements-intro|
+  2. Configuration.......................................|requirements-config|
+  3. Functions........................................|requirements-functions|
+
+==============================================================================
+INTRODUCTION                                              *requirements-intro*
+
+<doc/requirements.txt> is generated by <https://github.com/google/vimdoc>. See
+<README.md> for more information about installation and screenshots.
+
+==============================================================================
+CONFIGURATION                                            *requirements-config*
+
+                                           *requirements:g:requirements#cache*
+Completion cache path.
+Default: s:cache_dir . '/requirements.json' `
+
+                                           *requirements:g:requirements#items*
+Completion cache contents. For program.
+Default: s:items `
+
+==============================================================================
+FUNCTIONS                                             *requirements-functions*
+
+coc#source#requirements#init()                *coc#source#requirements#init()*
+  https://github.com/neoclide/coc.nvim/wiki/Create-custom-source
+
+coc#source#requirements#complete({opt}, {cb})
+                                          *coc#source#requirements#complete()*
+  https://github.com/neoclide/coc.nvim/wiki/Create-custom-source
+
+  Completion pip option and PYPI package names. Note: completion PYPI package
+  names need install pip-cache from PYPI, and run `pip-cache update` first to
+  generate cache.
+
+requirements#update_cache()                      *requirements#update_cache()*
+  Update cache.
+
+
+vim:tw=78:ts=8:ft=help:norl: