[feature] - Write docs skeleton and automation (spylang#349)

<img width="2448" height="1284"
alt="531486451-280bdf49-1333-4201-bc71-1c57165d7d83"
src="https://github.com/user-attachments/assets/eae09e4b-3de7-4ec8-a289-9b8b603dc01e"
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### Relating Issue: [314](spylang#314)

### Description:
This pull request sets up infrastructure for future documentation of
Spylang. Please refer to
[issue](spylang#314) for more context.

In this PR, it compose of 

1. `.github/workflows/docs.yml` : the github action file for building
Mkdocs with mike (document versioning) and push compiled document to
[Spylang Documentation Repository](https://github.com/spylang/docs#) for
github page (or any static web host in the future).

2. `docs/mkdocs.yml` : such a Mkdocs configuration file for static site
generation of Spylang documentation

3. `docs/src/index.md` : is a example document file.

4. `readme.md` : I modified readme file to describe our documentation.

### Must do
Before merging this PR, you must;

1) ✅ Generate ssh key file (public-private key). 
2) ✅ Attach (1) private key to **deployment key**
documentation repository
[spylang/docs](https://github.com/spylang/docs#). Now I assume,
@antocuni already created it.
3) ✅ Create **secret** variable for github action
pipeline in [spylang/spy](https://github.com/spylang/spy) repository.
It's there as `SPYLANG_DOCS_DEPLOY_KEY`.
4) ✅ Create **environment variable** of
[spylang/docs](https://github.com/spylang/docs#) origin's url at
[spylang/spy](https://github.com/spylang/spy). @kanin-kearpimy did it as
`SPY_DOCS_REPOSITORY`.
5) 🔴 Enable `github page` in
[spylang/docs](https://github.com/spylang/docs#) to host our static
document.
6) 🔴 Configure `github-pages` in
[spylang/docs](https://github.com/spylang/docs#) environments to allow
write.

#### Everything is all set!

Everytime we would like to update our documentation version, we could do
**git tag/release** in such pattern: v[major].[minor].[patch] such as
v0.0.0. Such `0.0.0` will be our version of document.