documentation.tex assume to be a specific location relative to Cactus root directory

Issue #2638 new
Roland Haas created an issue

Currently the LaTeX documentation files must include a relative path to `cactus.sty` like so:

```\usepackage{../../../../doc/latex/cactus}
```

which, assuming the “typical” directory layout, accidentally works both when calling `pdflatex` directly (and manually) on the LaTeX input file in the source code location and via the various `-ThornDoc` and `-ThornGuide` make targets.

This is however fragile and fails if e.g. the thorn lives in a git repository of its own so that the directory layout is:

```Cactus
|
+- repos
|
+- ThornRepo
|
+- doc
|
+- documenation.tex
```

rather than the expected

```Cactus
|
+- repos
|
+- ArrRepo
|
+- Thorn
|
+- doc
|
+- documenation.tex
```

or (the initially envisioned one assumes):

```Cactus
|
+- arrangements
|
+- ArrangeMent
|
+- Thorn
|
+- doc
|
+- documenation.tex
```

One way to handle this is to use conditionals to try both locations:

```\IfFileExists{../../../doc/latex/cactus.sty}
% then
{\usepackage{../../../doc/latex/cactus}}
% else
{\usepackage{../../../../doc/latex/cactus}}
```

which is somewhat cumbersome and only works for those two locations so must be adjusted for each thorn. Another, likely the officially suggested way, is to the `TEXINPUTS` such that `cactus.sty` is directly found. E.g. something like:

```export TEXTIPUTS=\$TOP/doc/latex/:
```

in the Makefile and then use `\usepackage{cactus}` in `documentation.tex`. This however means one can no longer run `pdflatex` directly on the LaTeX sources. There is, since TeX does not actually itself do the searching, no way to set `TEXINPUTS` or a similar thing in TeX files it seems.

`\IfFileExists` may rely on LuaTeX features (not sure) and one may even have to resort to pure TeX for say tex4ht to work (https://tex.stackexchange.com/questions/98203/can-i-test-if-a-file-exists).