reference docs

[robtaylor]

• docs: add reference-level documentation.
• docs/memory: add guide-level documentation.
• csr.bus: fix incorrect flow of element members of register signatures.
• docs/csr/bus: add guide-level documentation.
• docs/_static/custom.css: sync with amaranth upstream.
• docs/csr/reg: add guide-level documentation.
• docs/gpio: add documentation.
• csr.reg: remove Register._doc_template attribute.