To test compilation of optional ESP8266 modules, these modules are enabled for the `esp8266-olimex-mod` board when compiled in CI. The use of some of these optional modules depend on the use of other modules.
Fixes various issues in documentation:
- replaces HTML anchor tags in markdown headings by markdown heading IDs.
- replaces HTML linebreaks by newline commands
- replaces \htmlonly commands for image resizing by the width argument in \image commands
- replaces
- uses more unique identifiers for references
- uses stable references for driver documentation
- fix broken links to external HTML documents
- replaces the TOC link in headers by a back to TOC link below each section
- replaces ``` fenced code blocks by ~~~
