DOC: Clean up redirects & dead links
Ran `sphinx-build -b linkcheck` and fixed some redirects and dead links

> 

we could add a check for this (after everything is fixed) > > 
> 
> we could add a check for this (after everything is fixed)

Yup xref https://github.com/pandas-dev/pandas/issues/45409 @jreback ok to backport this? (it made a change to the 1.4.0 release notes so they are out of sync or could just sync the notes but would maybe be good to have the broken links fixed on the next release of the docs) oh sure any of the doc ones are fine @meeseeksdev backport 1.4.x Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
git checkout 1.4.x
git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
git cherry-pick -m1 ca5d9ee9d1eae7c642634413cb22092e0c9334ea
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
git commit -am 'Backport PR #45408: DOC: Clean up redirects & dead links'
```

4. Push to a named branch:

```
git push YOURFORK 1.4.x:auto-backport-of-pr-45408-on-1.4.x
```

5. Create a PR against branch 1.4.x, I would have named this PR:

> "Backport PR #45408 on branch 1.4.x (DOC: Clean up redirects & dead links)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the `Still Needs Manual Backport` label once the PR gets merged.

If these instructions are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 conflict was with #45456 which was already backported.