docs: no hosted pricing copy in the OSS docs tree

[DmitriyG228]

The OSS docs tree is a working place — no hosted pricing copy in it (founder, 2026-09-05).

Removed: the hosted welcome-credit sentence ($5 / about 16 hours / $0.30/hr, with the pricing link) from docs/docs/quickstart.mdx and docs/docs/how-to/send-a-bot.mdx, and the "— free credit, no card" tail from the shared get-a-key line on the other pages that carry it. The link to the hosted account page stays; the offer itself lives on vexa.ai and the hosted docs surface (Vexa-ai/docs).

Supersedes the earlier shape of this PR ($5 → $2). Closes #1540 as no longer applicable to this tree. README.md line 63 carries the same sentence and is outside the docs tree — untouched here.

Contribution rights

• Independent: I created this contribution, or otherwise have the right to submit it
  under Apache-2.0, and it is not owned or controlled by an employer, client, or other entity.
• Employer/client authorization required: an employer, client, or other entity owns or
  may control this contribution. I am requesting Vexa's private corporate-authorization process.
• Unsure: I need a private rights review before merge.

Every commit must also carry the contributor's own DCO Signed-off-by line. Selecting the
independent path means no individual CLA is required. Corporate and unsure paths do not stop
technical review, but they do block merge until resolved.

[github-actions]

🃏 Merge card — #1574

check		what it needs
Value	❌	missing state: value-signed (the value sign-off)
Diff	✅	maintainer self-review — @DmitriyG228 holds the commit bit (no separate non-author review required)

Not mergeable yet — every row above must be accepted before merge (choke point 1). Fill in what's ❌ above, then this clears automatically.

_{How a PR reaches merge: the merge bar.}