docs: (LaTex) adapted documenation to be better convertible

Rearrange old main manual section heading, so that pandoc/sphinx don't get confused.
Removed outdated configuration files.
Changes some formating.
Fixes the backward-compatibility reference.
Merge storage-backends.tex and storage-backend-droplet.tex, so that it can be better handled by sphinx.
Enable "make check" again, so that the LaTex code can be checked for errors.

docs/manuals/en/main/Makefile

@@ -146,7 +146,7 @@ check: bareos-manual-main-reference.pdf
 	@# hyperlink: multiline: pdfTeX warning .* has been reference$d but does not exist
 	@# defDirective multiline: Warning: defDirective{Dir}{Director}{PluginNames} has no matching resourceDirective and will be ignored.
 	@# pdfTeX warning (dest): name{directiveDirJobMigration\040Time} has been referenced but does not exist, replaced by a fixed one
-	@#if ! grep "LaTeX Warning: Hyper reference .* undefined on input\|LaTeX Warning: There were undefined references.\|pdfTeX warning .* has been reference\|Warning: defDirective\|pdfTeX warning (dest): name" $(LOG); then echo "CHECK: OK"; true; else echo "CHECK: FAILED: undefined references"; false; fi
+	@if ! grep "LaTeX Warning: Hyper reference .* undefined on input\|LaTeX Warning: There were undefined references.\|pdfTeX warning .* has been reference\|Warning: defDirective\|pdfTeX warning (dest): name" $(LOG); then echo "CHECK: OK"; true; else echo "CHECK: FAILED: undefined references"; false; fi
 
 clean:
 	if [ -e html/images/ ]; then rm -f html/images/* && rmdir html/images; fi