Silent payments page improvements #1127

Creating this issue for new & existing (pending comments from PR improvements to silent payments page:

• add relative URL (sorry I totally missed this when checking against checklist PR)
• improve text legibility in mobile version of illustrations
• consider a modal in send page mockup to handle address delta where the diff might be hard to read
• fix header-1200x630 image for name repetition
• mention 'find my transaction' as a way to quickly scan mempool before transaction gets into a block
• add link to silent payments hub
• add copy/warning about spark address visual similarity and correct error messaging (Discord topic)
• add list of challenges: detecting unconfirmed txns, scanning delays after long time etc.
• add note/warning regarding possible adverse effect on labels on syncing/scanning

Please assign this issue to me, I can tackle these and any (within reason) other items that come up after the page is public...