WEB: decommission dev.pandas.io
#### Location of the documentation

http://dev.pandas.io/

#### Documentation problem

Both http://dev.pandas.io/ and https://pandas.pydata.org/docs/dev/ exist, and it's confusing what the difference is / what each are for.

#### Suggested fix for documentation

- [x] Remove references to `dev.pandas.io` -> #39377
- [ ] Ensure all content from http://dev.pandas.io/pandas-blog/ is on https://pandas.pydata.org/community/blog/
   - https://github.com/pandas-dev/pandas-blog/tree/master/content
   - https://github.com/pandas-dev/pandas/tree/master/web/pandas/community/blog
- [ ] Redirect `dev.pandas.io`