Add rename channels example

Author: cbrnr

docs/examples/images/rename_channels/delete_characters.png

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+# Renaming Channels
+
+We use [S001R06.edf](https://www.physionet.org/files/eegmmidb/1.0.0/S001/S001R06.edf?download) from the [EEG Motor Movement/Imagery Dataset](https://www.physionet.org/content/eegmmidb/1.0.0/) in this example. In this data set, all channel names contain trailing dots (for example, `C3..` instead of `C3`). These extra characters prevent MNELAB from correctly matching the channels to a standard montage (that is, predefined channel locations), so let's remove them from the channel names.
+
+To do this, select *Channels – Rename Channels...* and configure the dialog as follows to strip the dots from the end of the channel names:
+
+![Stripping characters](images/rename_channels/strip_characters.png)
+
+After clicking *OK*, the channel names are updated.
+
+Whereas the *Strip characters* option removes a specified character (or characters) from the beginning or end of the channel names (including multiple occurrences), the *Delete characters* option removes a specified number of characters. We can use the [Australia-Pre-ICA.edf](https://osf.io/4g3uw/?action=download) example file to demonstrate how this works. In this file, all channel names end with "-LE". We could use the *Strip characters* option as before, but we can also use the *Delete characters* option to remove the last three characters from the channel names, which achieves the same result:
+
+![Deleting characters](images/rename_channels/delete_characters.png)