Fix broken anchor links in the glossary

It seems like `mkdocs` now also validates the anchor links in the
documentation, and it was complaining about the broken links in the
glossary. This commit fixes the broken links by changing the anchor
names to lowercase and linking to the code where appropriate.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

docs/user-guide/glossary.md

@@ -61,7 +61,7 @@ components](#component-category) together. It can be connected to the public
 [grid](#grid), or be completely isolated, in which case it is known as an
 island.
 
-Components can be grouped into [assets](#Assets) and [devices](#Devices).
+Components can be grouped into [assets](#assets) and [devices](#devices).
 Assets are core components like generators or storage systems that are crucial from a business perspective,
 whereas devices are the supporting infrastructure essential for the functionality of these assets.
 
@@ -408,7 +408,8 @@ flowing into the component instead, this will be zero.
 
 In [microgrid](#microgrid) context sometimes used as the remaining difference
 between the [net consumption](#net-consumption) and the [battery](#battery)
-power, i.e. what we define as [grid power](#grid_power).
+power, i.e. what we define as
+[`grid().power`][frequenz.sdk.timeseries.grid.Grid.power].
 
 ### Residual Load