make instructions to fetch bridges more concise, link to the relevant re-organized section of the manual and mention the offline manual

closes #374 & please also see manual!120

1. I was reviewing this article with https://tb-manual.torproject.org/bridges/ since we recursively link to it. For changes to the manual, see manual!120
   I am moving much of the instructions to the manual (is already much detailed and I hope the restructuring of the sections as part of manual!120 might help).

2. Mentioning the offline manual with the reasoning that users who might be unable to follow the various links to the manual in this article here might find that useful (given they have TB installed of course 🙂 ).