Update "bring your own cert" docs to add -days option for cert expiry

[jflowgh]

Description of the change
Update docs for "bring your own certs" as per #1126

Benefits
Docs work for generating your own certs and sealing them.

Possible drawbacks
Need to pick a sane default expiry time for docs

Applicable issues
N/A

Additional information
N/A

[agarcia-oss]

Thanks for the contribution, @jflowgh ! The changes LGTM; We just need you to include the Sign-off comment in the commit as per the project's Contributing guidelines.

[josvazg]

A quick way to sign off one commit:

git commit --amend -s --no-edit
git push -f

[josvazg]

Change LGTM too, will approve once the DCO sign-off get sorted out.

[jflowgh]

OK the signed off message is there now. Will remember for next time.

[agarcia-oss]

LGTM, thanks!

[alemorcuq]

Thanks!