docs: new window functions

[soyeric128]

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Summary

added new window functions: #11148

• Closes #issue

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Name	Status	Preview	Comments	Updated (UTC)
databend	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 14, 2023 8:45am

[what-the-diff]

PR Summary

• Edited Documentation Links
  A hyperlink in the Python documentation was corrected to improve the process of navigating resources.

• Added Function Notes
  There were clarifications made to the CUME_DIST function's documentation, highlighting that its return value ranges between 0 and 1. Mention of the PERCENT_RANK was also introduced.

• Removed Function Headings
  The headers related to certain functions in 'dense-rank.md', 'rank.md' and 'row-number.md' files were taken out for reasons not specified.

• Added New Function files
  Several new files were created to provide detailed descriptions and syntax for various functions including:

  1. FIRST_VALUE
  2. FIRST (Alias to FIRST_VALUE)
  3. LAST_VALUE
  4. LAST (Alias to LAST_VALUE)
  5. NTH_VALUE
  6. LEAD
  7. LAG
  8. NTILE
  9. PERCENT_RANK

• Updated Function Index
  The function index documentation was updated to include links to the newly added functions above.

These changes should enhance the documentation's completeness and accuracy, making it easier for users to navigate and understand the functions.