firmware documentation - axis_signal_gen_v6

[meeg]

• some tweaks to the top-level firmware page in RtD
• new RtD page for the axis_signal_gen_v6 IP

[lharnaldi]

Closing this in favor of #430, which folds SG-v6 documentation (and its content, corrected and expanded) into a larger restructuring of docs/ → docs/source/ from openquantumhardware/qick_internal's fix-tprocv2-trm-docs branch. That branch also targets the old flat docs/ layout this PR modifies, which #430 replaces entirely, so keeping both open would just produce a conflict.

A few corrections that landed on top of this content along the way, in case useful elsewhere: the Python usage examples here used a gen.set_freq()/gen.load_waveform()/gen.set_pulse() API that doesn't exist on the actual driver — replaced with the real declare_gen()/add_envelope()/add_pulse()/pulse() workflow, verified against qick.qick_asm/qick.asm_v2. Also the ..image:: directive was missing a space (so it silently rendered as plain text) and pointed at a PNG that was never actually committed — dropped the reference rather than leave a broken link.

Thanks for starting this — closing it here rather than losing the diff.