feat(start-here): add headings and update illustrations

[aigerimu]

closes #1712
closes #1711
closes #1681
closes #1710

[github-actions]

No documentation issues detected.

[novusnota]

Backed up removed ADNL paragraphs into the comment under a corresponding issue: #1035 (comment)

P.S.: Bouncer complains because of more than three issues linked (4), but that's ok here due to the small scope and connectivity of those, so we may proceed as is.

[it09]

• new diagrams look good - thank you!
• new sections make it easier to read
• [Start here] Remove advanced illustrations #1712 where did we move to advanced illustrations from start here? we still need to publish them in the advanced sections
• [Start here] Remove ADNL/TCP/liteserver deep dive that confuses beginners #1681 there is still mentioning of ADNL/liteserver in the network communication section - do you think its too advanced for someone who reads about TON for a first time?
• [Start here] Restructure “Start here” into a clear beginner onboarding flow (sections + explanations) #1685 this ticket is not addressed here, we moved context around to make it more readable, but its still not a beginner-friendly guide (I advise to address this ticket in another PR)

[aigerimu]

• new diagrams look good - thank you!
• new sections make it easier to read
• [Start here] Remove advanced illustrations #1712 where did we move to advanced illustrations from start here? we still need to publish them in the advanced sections

Backed up removed illustration into the corresponding issue: #1826

• [Start here] Remove ADNL/TCP/liteserver deep dive that confuses beginners #1681 there is still mentioning of ADNL/liteserver in the network communication section - do you think its too advanced for someone who reads about TON for a first time?

It might be a bit advanced; but it provides the full picture of how nodes interact, so let's keep it. Also, with clear headers, beginners can just skip that part if it feels too much.

• [Start here] Restructure “Start here” into a clear beginner onboarding flow (sections + explanations) #1685 this ticket is not addressed here, we moved context around to make it more readable, but its still not a beginner-friendly guide (I advise to address this ticket in another PR)

Ok, then won't address this issue here, and will reopen this issue of adding headers and assign to it