doc: Rework internal and external links

[maflcko]

Some minor changes:

• Move Bitcoin Core download link to the very top. Reason: The download link has nothing to do with the section that explains Bitcoin. Also, anyone quickly looking for the download link will find it faster.
• Add a new link to the doc folder. Reason: Apart from the documentation that is shipped with the binary software, the doc folder is the primary location for Bitcoin Core related documentation.
• Remove dead link to pdf. Reason: The pdf can be trivially found by asking a search engine.
• Remove reference to "build server". Reason: There is no "build server". The CI system is explained in the next sentence in detail.
• Remove dead? link to translation mailing list. Reason: The translation process is explained in doc/translation_process.md, no need to explain it in detail in the main readme.

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Conflicts

Reviewers, this pull request conflicts with the following ones:

• doc: fix broken link to whitepaper in README #20976 (doc: fix broken link to whitepaper in README by ben-kaufman)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[laanwj]

ACK 77772a1

Remove dead? link to translation mailing list. Reason: The translation process is explained in doc/translation_process.md, no need to explain it in detail in the main readme.

Good find. I can't even remember anymore when we used that list.

[RiccardoMasutti]

ACK 77772a1