DOC: append deprecation
- [ ] adds to documentation for #35407 

Sorry, hurried too much, changed it to pandas.concat and especially added it to the Series.append method too @gesoos if you can update > @gesoos if you can update

Updated it, tried linking to the deprecation note in the what's new, but I am not 100% sure the formatting is correct, so let me know and I will fix it @gesoos its not necessasry to add that link, not what i meant.

its failing: https://github.com/pandas-dev/pandas/runs/5012248628?check_suite_focus=true

just make this simple. Hello @gesoos! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2022-02-11 12:24:25 UTC @gesoos check https://pandas.pydata.org/docs/development/contributing_documentation.html#building-the-documentation and the examples using `--single` Sorry, this will take me a few more days, I need a new computer and it will only be available tomorrow. Thanks for the hint with the documentation building, was not aware that it's possible! @gesoos if you can update  I think the deprecation was adequately documented in the release notes https://pandas.pydata.org/pandas-docs/dev/whatsnew/v1.4.0.html#deprecated-frame-append-and-series-append and all other documentation was updated appropriately in #44539. The only thing missing is the deprecated tag for the the method itself?

having `.. deprecated:: 1.4.0` should be sufficient as users can refer to the release notes for 1.4? > I think the deprecation was adequately documented in the release notes https://pandas.pydata.org/pandas-docs/dev/whatsnew/v1.4.0.html#deprecated-frame-append-and-series-append and all other documentation was updated appropriately in #44539. The only thing missing is the deprecated tag for the the method itself?
> 
> having `.. deprecated:: 1.4.0` should be sufficient as users can refer to the release notes for 1.4?

sounds good - @gesoos want to update it to this? @MarcoGorelli I can update it, but I cannot build the documentation. If you have a more detailed description on how this should work than what can be found [here](https://pandas.pydata.org/docs/development/contributing_documentation.html), please let me know... There is a known [issue with the Dockerfile](https://github.com/pandas-dev/pandas/issues/45807), which I already noticed and am using the suggested fix, but it throws `ModuleNotFound` at every corner, for:
- docutils
- numpy
- pytz
- dateutil
- pandas._libs.interval (which is where I gave up) Hey - I'd suggest following the conda instructions we should do this for 1.4.1 sorry @gesoos to mess you around. This was covered in our monthly dev meeting and agreed that a one-liner would be helpful

have updated this PR as release is planned for tomorrow.

![image](https://user-images.githubusercontent.com/13159005/153584791-6d28c9fd-2bd0-4586-8e49-4ab018eea965.png)
 merge when ready > merge when ready

am just rebuilding docs after flake8 fixup to be sure. Thanks @gesoos 