Add datetimeindex accessors to timeseries documentation (timeseries.rst)

[mikebailey]

All of the timestamp/datetimeindex accessors are not in the timeseries documentation (they currently are only referenced in the api documentation). It would be great to have a section on these accessors:

.year
.month
.day
.hour
.minute
.second
.microsecond
.nanosecond
.date
.time
.dayofyear
.weekofyear
.week
.dayofweek
.weekday
.quarter
.is_month_start
.is_month_end
.is_quarter_start
.is_quarter_end
.is_year_start
.is_year_end

Additionally, some of these accessors (e.g. weekofyear) are not frequency aware and users should be alerted to that fact.

[jorisvandenbossche]

Would you be able to do a PR for this? Indeed very welcome!