"Upgrading": add more topics (#141)

Adding https://github.com/SAP-docs/sapui5/blob/main/docs/03_Get-Started/don-t-use-deprecated-or-experimental-features-a8bd1a8.md and https://github.com/SAP-docs/sapui5/blob/main/docs/03_Get-Started/best-practices-for-developers-28fcd55.md.

Co-authored-by: Günter Klatt <57760635+KlattG@users.noreply.github.com>

docs/02_Read-Me-First/upgrading-9638e4f.md

@@ -10,7 +10,10 @@ Before upgrading to a newer SAPUI5 version, you must generally first ensure that
 -   [Use Only Public APIs](../03_Get-Started/use-only-public-apis-b0d5fe2.md)
 -   [Coding Issues to Avoid](../05_Developing_Apps/coding-issues-to-avoid-3877872.md)
 -   [The Owner Component](../04_Essentials/the-owner-component-a7a3138.md)
+-   [Don't Use Deprecated or Experimental Features](../03_Get-Started/don-t-use-deprecated-or-experimental-features-a8bd1a8.md)
 -   For SAPUI5 applications based on SAP Fiori elements: [Read Before Extending a Generated App](../06_SAP_Fiori_Elements/read-before-extending-a-generated-app-d9c146a.md)
 
+See also related guidelines from [Best Practices for Developers](../03_Get-Started/best-practices-for-developers-28fcd55.md).
+
 Afterwards, refer to the following version-specific sections that describe what else you have to consider: