Add a pyserial migration guide

[puddly]

A small guide to cover 99% of migration cases. If you're not subclassing anything and just use public APIs, this will be more than enough to migrate an existing project.

[Copilot]

Pull request overview

Adds a new documentation page to help users migrate from pyserial/pyserial-asyncio to serialx, and updates the docs build configuration to support the new page’s Sphinx directives.

Changes:

• Add a new “Migrating from pyserial” how-to guide covering packaging, common API renames, and async modem-pin usage.
• Wire the new guide into the docs toctree.
• Enable sphinx_design and add docs-only dependencies needed for the new directives; minor docstring warning formatting tweaks for deprecated aliases.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
serialx/common.py	Adjusts deprecated-alias docstring warning formatting for consistency.
pyproject.toml	Adds docs optional deps (sphinx-design, sphinx-autobuild) required for the new docs content/workflow.
docs/index.md	Adds the new migration guide to the “How-to” toctree.
docs/how-to/pyserial-migration.md	New migration guide documenting common rename patterns and async best practices.
docs/conf.py	Enables the sphinx_design Sphinx extension used by the new guide.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.77%. Comparing base (9d90d73) to head (095ab53).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #71   +/-   ##
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  Coverage   90.77%   90.77%           
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  Files          21       21           
  Lines        3348     3348           
=======================================
  Hits         3039     3039           
  Misses        309      309           

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