Change solar power density from 1.7 to 4.6 MW/km2

[pz-max]

Closes #361

Changes proposed in this Pull Request

Change the socio-technical installable solar power density from 1.7MW/sqkm to 4.6MW/sqkm.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changes in configuration options are added in all of config.default.yaml, config.tutorial.yaml, and test/config.test1.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[pz-max]

Approved by @davide-f . Different to Europe, in Africa we have many areas that do not have socio-technical constraint such deserts, hence, the 1.7 MW/sqkm might be overly pessimistic (average solar density 50 MW/sqkm, max ~100sqkm).

This PR suggest that the socio-technical acceptable solar power density for all eligible areas is at 4.6 MW/sqkm. Another issue (with future PR) #362 aims to let the power density change depending on the land cover type.