Add examples/case studies to the R Dev Guide

[hturner]

We now have several examples from R Dev Days that could be added as cases studies to the R Dev Guide. Since they usually cut across R/C files, help files and test files, I think it would make sense to put these in a separate Case Studies chapter.

The text should include properly marked up diffs, e.g.

```diff
- plot(cars)
+ plot(pressure)
  title("Example")
```

which is typeset as

- plot(cars)
+ plot(pressure)
  title("Example")

Here are some good examples, grouped by difficulty:

Novice R

• (R and help files): Bug 18773 - grid::dataViewport() has minor bugs in error messages and help

Intermediate R:

These were all covered in my covered in my R-Ladies Melbourne talk
- (R and help files, requires a bit more knowledge about deprecation) Bug 17835 - Removing master/slave terminology from tcltk
- (R and help files) Bug 7084 - text(x, y, labels) - recycling problems
- (R, Rd, test files) tools::toTitleCase() - first PR is straight-forward: r-devel/r-svn#172, some additional issues picked up and patched which don't need to be included in case study

Novice C:

    - (C, change in error messages)  [Second-last step towards migrating from legacy (Kb, Mb, ...) to IEC (KiB, MiB, ...) units](https://github.com/r-devel/r-dev-day/issues/28)
    - (C, Rd, tests) [3-digit hex colors](https://github.com/r-devel/r-project-sprint-2023/issues/74) 

• Relatively simple C issues - covered in @EllaKaye's C for R Users talk
  • (C and test files): Bug 8934 - Irregularity in stem() display - covered in @EllaKaye's C for R Users talk
    • (C and test files, also fixed for array case, which was a simpler patch): Bug 15027 - max.print is ignored for matrix headers
  • (R, C, Rd, tests): Add 'rot' argument to glyphInfo() to define rotation angle of shapes within a font
    • (relatively understandable R and C changes, but patch not so well shared): Add an argument to pdf() that turns off the inclusion of a date

[b-s-code]

I will look into this.