Fix the section "Installation from sources"

An instruction about the Zabbix installation from git.zabbix.com is missed for all versions.
Users can't install Zabbix using https://www.zabbix.com/documentation/current/manual/installation/install.
Source code from git doesn't content a script ./configure and SQL scripts for creating database schema.

I suggest to add the instructions about "./bootstrap.sh" script and move the text "If installing from SVN, it is required to run first:
$ make dbschema" to the page https://www.zabbix.com/documentation/current/manual/appendix/install/db_scripts.