*: refactor transaction documents (#2088)

[sre-bot]

cherry-pick #2088 to release-2.1

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What is changed, added or deleted? (Required)

Original PR: #1695 (closed)

This PR refactors and updates transaction-related documents, including:

• Split and merge the current Optimistic Locking best practice doc into existing Transaction Documents (because it is not a best practice doc, rather, it is a compilation of existing content on transactions.)
• The Transaction Overview doc include content applicable to both Optimistic Transaction and Pessimistic Transaction
• The previous Transaction Model is now changed to Optimistic Transaction

Version differences: pingcap/docs-cn#2255

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version, including v4.0 changes for now)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

If you select two or more versions from above, to trigger the bot to cherry-pick this PR to your desired release version branch(es), you must add corresponding labels such as needs-cherry-pick-3.1, needs-cherry-pick-3.0, and needs-cherry-pick-2.1.

What is the related PR or file link(s)?

• This PR is translated from: *: refactor transaction documents docs-cn#2198
• Other reference link(s):
  TiKV | Distributed transaction
  Shirly's Blog | Transaction in TIDB
  Transaction in TiDB
  Transaction Model | TiDB Documentation
  Automatic retry and transactional anomalies caused by automatic retry

[yikeke]

LGTM

[sre-bot]

/run-all-tests