*: convert remaining sequence diagrams from PNG to Mermaid

[kennytm]

What is changed, added or deleted? (Required)

The remaining images which are obviously sequence diagrams are converted to mermaid. Totally 8 images are found from 4 pages.

• br/br-log-architecture.md ("BR log backup process design" & "Point-in-time recovery process design")
• br/br-snapshot-architecture.md ("snapshot backup process design" & "snapshot restore process design")
• dm/feature-shard-merge-pessimistic.md ("shard-ddl-flow")
• pessimistic-transaction.md ("TiDB pessimistic transaction commit process" & "Pessimistic transactions in TiDB" & "Pipelined pessimistic lock")

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

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

• This PR is translated from:
• Other reference link(s): Part of Support for mermaid #15560

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[gemini-code-assist]

Code Review

This pull request replaces static images with Mermaid sequence diagrams in the documentation for BR log backup, snapshot backup and restore, DM pessimistic shard merging, and pessimistic transactions. These changes improve the maintainability of the architectural diagrams. I have no feedback to provide.