docs: Added docs for BDF fonts (#6398)

Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

docs/overview/font.rst

@@ -342,6 +342,85 @@ Example
    /*Free the font if not required anymore*/
    lv_binfont_destroy(my_font);
 
+Use a BDF font
+**************
+
+Small displays with low resolution don't look pretty with automatically rendered fonts. A bitmap font provides
+the solution, but it's necessary to convert the bitmap font (BDF) to a TTF.
+
+Convert BDF to TTF
+------------------
+
+BDF are bitmap fonts where fonts are not described in outlines but in pixels. BDF files can be used but
+they must be converted into the TTF format via mkttf. This tool uses potrace to generate outlines from
+the bitmap information. The bitmap itself will be embedded into the TTF as well. `lv_font_conv <https://github.com/lvgl/lv_font_conv/>`__ uses
+the embedded bitmap but it also needs the outlines. One could think you can use a fake MS Bitmap
+only sfnt (ttf) (TTF without outlines) created by fontforge but this will not work.
+
+Install imagemagick, python3, python3-fontforge and potrace
+
+On Ubuntu Systems, just type
+
+.. code:: bash
+
+    sudo apt install imagemagick python3-fontforge potrace
+
+Clone mkttf
+
+.. code:: bash
+
+    git clone https://github.com/Tblue/mkttf
+
+Read the mkttf docs.
+
+Former versions of imagemagick needs the imagemagick call in front of convert, identify and so on.
+But newer versions don't. So you might probably change 2 lines in potrace-wrapper.sh.
+Open potrace-wrapper.sh and remove imagemagick from line 55 and line 64.
+
+line 55
+
+.. code:: bash
+
+    wh=($(identify -format '%[width]pt %[height]pt' "${input?}"))
+
+line 64
+
+.. code:: bash
+
+    convert "${input?}" -sample '1000%' - \
+
+It might be necessary to change the mkttf.py script.
+
+line 1
+
+.. code:: bash
+
+    #!/usr/bin/env python3
+
+Example for a 12px font
+-----------------------
+
+.. code:: bash
+
+    cd mkttf
+    ./mkttf.py ./TerminusMedium-12-12.bdf
+    Importing bitmaps from 0 additional fonts...
+    Importing font `./TerminusMedium-12-12.bdf' into glyph background...
+    Processing glyphs...
+    Saving TTF file...
+    Saving SFD file...
+    Done!
+
+The TTF TerminusMedium-001.000.ttf has been created from ./TerminusMedium-12-12.bdf.
+
+Create font for lvgl
+
+.. code:: bash
+
+    lv_font_conv --bpp 1 --size 12 --no-compress --font TerminusMedium-001.000.ttf --range 0x20-0x7e,0xa1-0xff --format lvgl -o terminus_1bpp_12px.c
+
+:note: use 1bpp because we don't use anti-aliasing. It doesn't look sharp on displays with a low resolution.
+
 Add a new font engine
 *********************