[erl_docgen] Update due to introduction of Unicode
[erl_docgen] Use DejaVu fonts when generating PDF

A few examples in lib/stdlib/doc/src/unicode_usage.xml make use of
Cyrillic characters.
Fix some FOP warnings

Examples overflowing the width of PDF pages have been fixed.

The remaining warnings are due to table cells, and require more
work.
Prepare OTP files for Unicode as default encoding
Implement ./otp_build configure --enable-silent-rules

With silent rules, the output of make is less verbose and compilation
warnings are easier to spot. Silent rules are disabled by default and
can be disabled or enabled at will by make V=0 and make V=1.
Merge branch 'nox/enable-silent-rules/OTP-10726'

* nox/enable-silent-rules/OTP-10726:
  Implement ./otp_build configure --enable-silent-rules
[erl_docgen] Fix pdf indentation bug for tagged lists
[erl_docgen] Fix dtd error for type_desc tag
[erl_docgen] Add xmllint target for the documentation and fix some DTD errors
[erl_docgen] Update vsn.mk
Update copyright years
Prepare release