DOC: cross reference similar `Series.unique` and `Series.drop_duplicates` methods
- [x] closes #xxxx (Replace xxxx with the Github issue number)
- [N/A] [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
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [N/A] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [N/A] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Personally, I found `unique()` before I found `drop_duplicates()`, even though the latter is closer to what I wanted to achieve. As discussed in https://github.com/pandas-dev/pandas/issues/1923#issuecomment-515465111, cross-referencing these methods would help with discovering the desired method.

Closes #1923
Docs successfully build locally:

![Series.drop_duplicates() documentation linking to Series.unique() documentation](https://user-images.githubusercontent.com/247555/172433702-9ae431b4-74c0-4eb4-847b-73cde8cb41f6.png)


![Series.unique() method documentation including a link to Series.drop_duplicates()](https://user-images.githubusercontent.com/247555/172433491-a9636ed9-8ddc-4f9c-a212-76ba7e9fee5d.png)

 Thanks @tswast 