docs/audiowmark.md: describe general architecture

Squashed history:
* Add usage as reported by --help
* Add `audiowmark get` overview
* Add graph to describe components
* Describe `audiowmark add` command usage
* Add comment with build commands and notes
* Fix basic HTML layout
* Move "Modulation Frame Generator" into separate chart
* Colorization adjustments
* Explode conv_encode and randomize_bit_order
* Fix block label
* Add prepare_frame_mod, UpDownGen, mark_data
* Describe mark_sync and frame_pos
* Elaborate ab_generators
* Describe gen_mix_entries and --linear
* Describe mark_data
* Describe init_frame_mod_vec
* Properly layout ab_generators
* Describe R1…R6 in detail
* Elaborate on architecture, encoding and PRNG
* Describe blocks and AES/CTR
* Adjust layout and paper size
* Add coarse extraction chart
* Add frame_pos UpDownGen sync_finder fft_range BlockDecoder
* Describe init_up_down
* Add fft_analyzer, sync_fft_256, sync_decode, search_approx
* Stretch graph
* Document sync_select_by_threshold
* Split SyncFinder and wmget graphs, refine wmget
* Refine FFT and BlockDecoder details, sync_finder output
* Fix all arrows and edges
* Fix "Extracting Audio Watermarks" titles and subtitles
* Describe refined search, document refining feedback
* Sketch BlockDecoder functionality
* Sketch ClipDecoder functionality
* Adjust decoder fft_range
* Document conv_decode_soft()
* Complete the BlockDecoder documentation
* Add bit vector description
* Document ClipDecoder
* Add decoder timing information
* Adjust ClipDecoder color
* Summarize synchronization detection
* Describe zero padding in Mode::CLIP
* Document result set printing
* Maximum of 5 sync positions applies to ClipDecoder
* Describe detection decision for A/B-Block
* Add prose about watermark detection and extraction
* Describe "Experimental Speed Detection"
* Fix digraph titles and spelling in a few places
* Adjust titles + boxes in "Modulation Frame Generator"
* Adjust titles + boxes in "audiowmark add"
* Adjust Power Measurement border
* Adjust titles + boxes in "Synchronization Position Finder"
* Add line break to improve digraph width
* Small wording fixups
Signed-off-by: Tim Janik <timj@gnu.org>