docs: fix broken "How to use" links in payment methods

[21Mill]

What does this PR do?

Fixes #2620

The "How to use" link on the payment methods pages pointed to docs/how-to-use.md,
a file that no longer exists, so it returned a GitHub 404. While fixing it I found
the three translations were broken as well:

Page	Before	After
01-best-practices/02-payment-methods.md	blob/main/docs/how-to-use.md — 404 (the reported one)	/read/en/
fr/…	blob/main/docs/fr/how-to-use.md — 404 too	/read/fr/
es/…	right path, wrong org casing (Robosats)	/read/es/
pt/…	linked to the Spanish tutorial	/read/en/

They now point at the rendered site pages rather than GitHub blob URLs. That matters
beyond the 404: the tutorials reference their screenshots with site-absolute paths
(/assets/images/how-to-use/…), which the blob view resolves against github.com,
so a blob link would land the reader on a page with every screenshot broken. The
permalinks used here (/read/en/, /read/es/, /read/fr/) are the same ones the
sidebar navigation already uses. pt falls back to /read/en/ because there is no
Portuguese tutorial.

Also fixed a dead image while in there: the Satoshi's Journal "Robo Garage" header in
the English and French tutorials (i0.wp.com/satoshisjournal.com/…/robosats.jpg)
returns 404, and so does the linked article — it is gone from the site entirely, not
just moved. The image is now vendored under assets/images/how-to-use/ (recovered
from the Wayback snapshot, 814×458, 96K) and the article link points at its Wayback
snapshot so the reference stays usable.

Verified: all 32 local image paths referenced by the tutorials resolve to files in the
repo, the three target permalinks exist, and no reference to i0.wp.com remains.

Not included: _data/navigation_pt.yml:100 links to /read/pt/, which does not exist
either — happy to fold that in or leave it for a separate PR.

Checklist before merging

• Install pre-commit and initialize it: pip install pre-commit, then pre-commit install. Pre-commit installs
  git hooks that automatically check the codebase. If pre-commit fails when you commit your changes, please fix the problems it points out.

[KoalaSat]

Ey @21Mill thanks for the PR! I was thinking maybe we should switch to another available video instead or relaying in the Internet Archive (with services down by the time I write this 😅

I would say the one still valid and widely used is https://www.youtube.com/watch?v=XW_wzRz_BDI

[21Mill]

The Internet Archive thing makes sense.

but the image itself is no longer served from archive.org. I vendored it into docs/assets/images/how-to-use/ so it loads from learn.robosats.org like every other screenshot in the tutorial. The only remaining Wayback dependency is the <a> around the heading, because the article itself is gone too.

The video is already embedded at _pages/tutorials/watch/videos-en.md:14, so adding it to /read/en/ would duplicate it across the two tutorial sections. Two options, happy with either:

1. Drop the dead entry. Remove the image + heading + blurb from how-to-use.md and how-to-use_fr.md, and drop the vendored .jpg. The BTC Sessions video already covers that ground on /watch/en/, one click away in the same sidebar nav.
2. Swap in the video as you suggested, embedded in /read/en/ the same way. Note /watch/fr/ doesn't carry this video, so the French page would end up with an English-only resource, fine by me if that's acceptable.

I'd lean towards 1, since it removes the archive.org link entirely without duplicating content. Let me know and I'll push the change.