Improve 'Cron (timer) triggers' page (#1319)

* Update cron-triggers.md

1. Deleted the 'Seconds' field from the 'Cron Expression Fields' table, as we support only five fields in the cron expression.

2. Moved 'Predefined Scheduling for Cron Jobs' and 'Fixed Interval Scheduling for Cron Jobs' to the 'Cron Triggers with Codefresh CLI' section (relevant only for legacy cron triggers), since our UI no longer supports them. Additionally, these schedules use six fields in the cron expression, which has been deprecated.

* Rename cron-timer-tab.png to cron-timer-tab-old.png

* Added new cron-time-tab.png

Updating the screenshot in the 'Create a Cron Trigger in UI' section (How-To, Step 5) to reflect the current support for five-field cron expressions in the UI. The previous screenshot incorrectly displays six fields.

* Delete images/pipeline/triggers/cron/cron-timer-tab-old.png

* Update cron-triggers.md

Removed 'Predefined Scheduling for Cron Jobs' and 'Fixed Interval Scheduling for Cron Jobs' sections

Author: maryiaCodefresh

_docs/pipelines/triggers/cron-triggers.md

@@ -157,7 +157,6 @@ The table below describes the fields you can define in a Cron expression.
 {: .table .table-bordered .table-hover}
 Field    | Mandatory | Allowed values  | Allowed special characters
 |----------   | ---------- | --------------  | --------------------------|
-Seconds      | No         | 0-59            | * / , -|
 Minutes      | Yes        | 0-59            | * / , -|
 Hours        | Yes        | 0-23            | * / , -|
 Day of month | Yes        | 1-31            | * / , - ?|
@@ -178,28 +177,6 @@ The table below describes the purpose of the special characters in a Cron expres
 |**Hyphen** (`-`) | Hyphens define ranges. For example, `9-17` would indicate every hour between 9am and 5pm inclusive.|
 |**Question mark** (`?`) | Question marks are instead of asterisks (`*`) to leave either day-of-month or day-of-week blank.|
 
-### Predefined scheduling for Cron jobs
-
-You can use one of several predefined schedules instead of a Cron expression.
-The table below describes the predefined schedules supported.
-
-{: .table .table-bordered .table-hover}
-|Predefined schedule                  | Description                  | Equivalent to|
-|-----                  | -----------                                | -------------|
-|@YEARLY (or @ANNUALLY) | Run once a year, midnight, Jan. 1st        | 0 0 0 1 1 *|
-|@MONTHLY               | Run once a month, midnight, first of month | 0 0 0 1 **|
-|@WEEKLY                | Run once a week, midnight on Sunday        | 0 0 0 ** 0|
-|@DAILY (or @MIDNIGHT)  | Run once a day, midnight                   | 0 0 0 ** *|
-|@HOURLY                | Run once an hour, beginning of hour        | 0 0 ** **|
-
-### Fixed interval scheduling for Cron jobs
-
-You can also schedule a job to execute at fixed intervals by adding `@every <interval>`. The <interval> is a string that represents the desired frequency.  
-For example, `@every 1h30m10s` would indicate a schedule that triggers every 1 hour, 30 minutes, 10 seconds.
-
->**NOTE**  
-The interval does not take the runtime of the job into account. For example, if a job takes three minutes to run, and it is scheduled to run every five minutes, it will have only two minutes of idle time between each run.
-
 ### Conditional triggers for Cron jobs
 
 Take a look at our Knowledge Base how-to: [Skip pipeline step if triggered by Cron job]({{site.baseurl}}/docs/kb/articles/skip-test-if-pipeline-is-triggered-with-cron/).