doc: improve documentation

- solve iframe size issue
- add content

Refs #23
1 job for master in 1 minute and 16 seconds (queued for 4 seconds)
Status Name Job ID Coverage
failed deploy #168187


Name Stage Failure
deploy Test
  invalid multibyte character in parser at line 7
Calls: <Anonymous> ... <Anonymous> -> parse_all -> parse_all.character -> parse
In addition: There were 18 warnings (use warnings() to see them)

Execution halted
The command '/bin/sh -c R -e 'rmarkdown::render(irmara:::app_sys("app/www/documentation/documentation.Rmd"))'' returned a non-zero code: 1
Connection to closed.
Cleaning up file based variables
ERROR: Job failed: exit code 1