docs: Document Merge Tokenizer Pipeline

[MoosaTae]

Why this PR

Why we need this PR?

Changes

• Write some changes here

Related Issues

Close #296

Checklist

• [ ] PR should be in the Naming convention
• [ ] Assign yourself in to Assigneees
• [ ] Tag related issues
• [ ] Constants name should be ALL_CAPITAL, function name should be snake_case, and class name should be CamelCase
• [ ] complex function/algorithm should have Docstring
• [ ] 1 PR should not have more than 200 lines changes (Exception for test files). If more than that please open multiple PRs
• [ ] At least PR reviewer must come from the task's team (model, eval, data)

[new5558]

Can you temporarily pause pytest by adding @pytest.fixture on a test case that has problem https://docs.pytest.org/en/6.2.x/fixture.html

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -72.23% :warning:

Comparison is base (6dd7d68) 94.15% compared to head (8608ab0) 21.93%. Report is 21 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #307 +/- ## =========================================== - Coverage 94.15% 21.93% -72.23% =========================================== Files 10 19 +9 Lines 291 1231 +940 =========================================== - Hits 274 270 -4 - Misses 17 961 +944 ``` | [Flag](https://app.codecov.io/gh/OpenThaiGPT/openthaigpt-pretraining/pull/307/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenThaiGPT) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/OpenThaiGPT/openthaigpt-pretraining/pull/307/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenThaiGPT) | `21.93% <ø> (-72.23%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenThaiGPT#carryforward-flags-in-the-pull-request-comment) to find out more. [see 29 files with indirect coverage changes](https://app.codecov.io/gh/OpenThaiGPT/openthaigpt-pretraining/pull/307/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenThaiGPT)

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