Add Using Cached Runner documentation

[fabi200123]

• Adds documentation for using cached GitHub action runners for Windows and Linux

Not to be merged before the following PRs are merged:

• garm-provider-common - #36
• garm-provider-common - #37

[gabriel-samfira]

I think we can refactor the performance_considerations.md doc and combine it with this. Or move the LXD specific docs to the LXD/inclus repos and replace the performance considerations with this. What do you think?

[fabi200123]

I think that a better approach here would be to move the LXD/Incus specific stuff to its provider repo. Then perhaps create a similar file for each provider repository. I think that would answer most people's questions regarding cached runners and perhaps even let them know they can use the cached runners