fix: say why scripts are refused and how a person adds one in the admin

[kevin1024]

The API never saves <script>, so that text an agent reads cannot talk it into adding code to a site. The guide and help only said scripts are stripped.

• Guide §21 (geekcli guide html) explains the reason and the workaround: a signed-in person opens the item's admin form, clicks the editor's code button and pastes the snippet. It lists the admin form for each kind of content.
• posts create, footers update help and the README say the same in a line.

The API is getting an admin_url on the warning for a removed script (cincpro/rg2 #5134); the CLI already prints warning messages, so the link shows up without a CLI change.