organization of dnsmasq configuration files - both DHCP and DNS

Define organization of dnsmasq configuration files and put documents in ics_doc. ics_dnsmasq repo is designed to be mapped to /etc/dnsmasq.d/ directory, and all of non configuration lines shall start with '#', which breaks rst (or markdown).
This document need to include:

• Organization of configuration files (directory and file schema/name, containts)
• Service (dnsmasq) global configurations
• Branch organization

Operational procedures (e.g. to register new hardware, to exchange hardware for maintenance) is planned to be developed in separated ticket (ref. INSTRM-70)

Based on current working configuration used in labs at JHU and LAM (#19c9f47):

• Store hostname to IP address bindings in hosts/ directory
• Store MAC address to hostname bindings in the top directory, which will be changed on hardware exchange for maintenance
• Have one set of files for above two bindings (in total two files) per one set of functional modules, e.g. BCU1, ENU1, PFI
• All dnsmasq global configuration (e.g. domain, log-dhcp) shall be in pfs.conf
• All external configuration (e.g. external DNS server) shall be in machine.conf

from [1], points 1,2 of 1st section, 1,2,4 of 2nd section. (5,6 of 1st section and 3 of 2nd section need to be defined in global configuration; 3,4 of 1st section is specific to JHU/LAM branch)

Also following restrictions/ways have confirmed, which need to be cared in configurations:

• addn-hosts=xxx is to set an additional directory for hosts files to be read by dnsmasq DNS service
• hosts files in addn-hosts directory can contain normal definition lines, such like "ipaddr name1 name2 ..."
• dhcp-host line can contain IP address, and can rely on hostname-ipaddr conversion defined in hosts files in addn-hosts
• (so, seems no way to have DNS records for not leased dhcp-host entry, if we don't take a way with hosts file)

*1 https://github.com/Subaru-PFS/ics_dnsmasq/blob/947663ffc3c63d4a9d9392cdc279627bffab6f95/PFS.README