Add doc introduction for INPUT parameter 'precision'

[denghuilu]

Details

Abacus support single precision SCF calculation in PW basis, we should add the related doc introduction

Have you read the online manual http://abacus.deepmodeling.com/en/latest/

• Yes, I have read the online manual.

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• Identify the specific section of the documentation with the issue.
• Investigate the issue and determine the root cause.
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• Update the documentation to address the issue, following the suggested improvement.
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• Test the updated documentation to ensure it is clear and accurate.
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