Test Angular UI extensibility docs & update screenshots

[hikalkan]

I moved the following docs from ABP Commercial to the ABP Framework documentation:

• https://docs.abp.io/en/abp/4.1/UI/Angular/Entity-Action-Extensions
• https://docs.abp.io/en/abp/4.1/UI/Angular/Data-Table-Column-Extensions
• https://docs.abp.io/en/abp/4.1/UI/Angular/Dynamic-Form-Extensions
• https://docs.abp.io/en/abp/4.1/UI/Angular/Page-Toolbar-Extensions

• Test these documents fully with the open source startup template, with version 4.1.0-rc.2.
• Update screenshots (current screenshots were taken with the lepton theme) with the basic theme.
• Also, test the main module entity extension system if it properly works with the open source identity module (no need to test database mapping side - we just need to understand the angular UI compability)

Note: Make changes in the rel-4.1 branch.