Font related changes.

[Sidicer]

Hello! I tried testing all possible fonts on SD1306 128x64 OLED display and noticed that there were some discrepancies with explanations and examples.

1. Because I already had an example written to cycle through all possible fonts and show all possible characters I wanted to add it to the repository.
2. I could not understand why ssd1306xled_font6x8_German, free_calibri11x12_cyrillic and free_calibri11x12_latin always showed garbage on the screen. After looking at an example it became obvious that the primary font still needs to be set, which means these three are not standalone fonts. For that I added warnings in the definitions.
3. German font was missing ß

Related changes:

• Added an example to showcase all possible fonts.
• Added ß character to ssd1306xled_font6x8_German table
• Added additional warnings in ssd1306_fonts.h

Tested on ATmega328P and ESP32-WROOM-32D using Arduino framework. Sadly was not able to test with espidf framework due not being able to resolve library includes.