DOC: Add to_julian_date to DatetimeIndex methods listing
First time contributor here.

- ~[ ] closes #xxxx (Replace xxxx with the GitHub issue number)~ No issue related
- ~[ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature~
- ~[ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).~
- ~[ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.~ Nothing new
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Add `to_julian_date` documentation entry to `pandas.DatetimeIndex`.

Currently, it is not listed nor can be found in the documentation: https://pandas.pydata.org/docs/reference/api/pandas.DatetimeIndex.html
`pandas.Timestamp` already has it linked https://pandas.pydata.org/docs/reference/api/pandas.Timestamp.to_julian_date.html
from the time it was added v0.14.0 https://pandas.pydata.org/docs/whatsnew/v0.14.0.html

I haven't read through the contributing section, please point out if there is something missing. If it looks okay, tell me to proceed with the whatsnew task.

Docs artifact for second commit (it works): https://github.com/pandas-dev/pandas/actions/runs/16895776565/job/47865152612?pr=62090
Thanks @echedey-ls 