Add information about timezones

[sirex]

Datetime values must always be provided with timezone information, for example:

2024-01-22T09:14:13+02:00

If timezone is not given, for example:

2024-01-22T09:14:13

Then level must be set to 2 (non-standard data).

Timezone for values without timezone can be set via prepare function:

tz('UTC')
tz('Europe/Vilnius')
tz('+02:00')
tz('-02:00')

If timezone is not given in value and timezone is not known, then level must be set to 2.