The Home Office guidance says the following:

Please note, you only need to list your absences where you have been out of the UK for more than 6 months in total in any 12 month period. You do not need to list any absences that were before your qualifying period of residence.
Short absences under six months or where they do not add up to more than six months in a 12 month period do not need to be listed on the PR form.

Absences before your chosen 5 year qualifying period need not be mentioned, regardless of length.

Although the form asks for absences from date of entry, it is not necessary to mention them if they were before your 5 year period. You can either answer YES, and then explain that no absences were over 6 months, or NO, and then explain that you have not been absent for more than 6 months in any 12 month period during your qualifying years.

See: Loss of PR status and ILR through absence from the UK.

Where absences have to be declared, they only need to be listed, there is no need to provide evidence.

A continuous absence of 6 months or longer will break the continuity of your qualifying period and reset the clock. Absences totaling over 6 months in any 12 month period will affect your application, if they fall within the qualifying period.

If you have various absences adding up to more than 6 months within a year, you may want to see if changing the starting month makes any difference. By splitting the absences over two years, you may end up with less than 6 months’ total absence within each year.

Absences after your 5 year qualifying period

Once you have accrued at least 5 years as a qualified person without absences of 6 months or longer, you would have acquired PR status, even if you haven’t got a document certifying it. You would only lose it if you stayed out of the UK for 2 years or longer. Absences under 2 years AFTER your 5 year period don’t affect your right to PR.

You will still need a full list of absences should you decide to apply for citizenship later on, but only for the relevant period (last 3 or 5 years).