Fix conflicting information about updating a package with a maintainer

In Matrix, someone pointed out an inconsistency in our docs that caused some confusion.

According to the documentation help.getsol.us/docs/packaging/update-an-existing-package, it says that if a package already has a maintainer, I should reach out to her/him directly to update the package. However, help.getsol.us/docs/packaging/procedures/maintainership/ states that I should either open a package update request or ask for permission here.

We should make the instructions on both pages consistent. Consider doing so with an include or separate page to keep all the text in one place, to avoid this confusion in the future.

Also, consider:

• What to do if the person has no Matrix info (only email)
• If the person has a Matrix account, detail what we mean by "reach out" - e.g. ping in the packaging channel (avoid suggesting DMs since some folks are iffy about random DMs from unknown people)

Compare the instructions at help.getsol.us/docs/packaging/update-an-existing-package and help.getsol.us/docs/packaging/procedures/maintainership/, then inspect the repository structure for their source files and any existing include pattern. Make the guidance consistent, clarify Matrix and email contact options without recommending unsolicited DMs, and verify both pages render with the same instructions.