Fix link to the IPython install page (#7783)

* Fix link text for IPython install page

* Update JupyterLab Night theme installation link

docs/source/migrating/custom-themes.md

@@ -8,7 +8,7 @@ This is for example the case for community contributed themes such as [jupyter-t
 
 Fortunately installing a custom theme for Notebook 7 is very easy. It is the same process as installing a regular extension.
 
-For example let's say you want to install the [JupyterLab Night](https://github.com/martinRenou/jupyterlab-night) theme. You can do so by running the following command:
+For example let's say you want to install the [JupyterLab Night](https://github.com/jupyterlab-contrib/jupyterlab-night) theme. You can do so by running the following command:
 
 ```bash
 pip install jupyterlab-night

docs/source/notebook.md

@@ -355,7 +355,7 @@ Specific plotting library integration is a feature of the kernel.
 ## Installing kernels
 
 For information on how to install a Python kernel, refer to the
-[IPython install page](https://ipython.org/install.html).
+[IPython install page](https://ipython.org/install).
 
 The Jupyter wiki has a long list of [Kernels for other languages](https://github.com/jupyter/jupyter/wiki/Jupyter-kernels).
 They usually come with instructions on how to make the kernel available