Replace call to Show Source button to view file on GitHub

There is no "Show Source" link in the documentation how to page,
but you can still see the raw rST bare formatting if you jump
over to the relevant page hosted on the GitHub repo.

Also:

- Plugin howto: Put command line entry on own line, easier to read
- FAQ - How many users: Write out number when under 10
Warn Windows developers to use short filepaths to reduce conflicts

Adds note in docs under "Set Up Your Environment" that long
filepaths will be troublesome if working on Windows platform.

Also fixes up some formatting of other points and italicises paths
for menu actions to help visually stand out from rest of text.

Updates a couple of URLs that have since 301 to new places.

Related to #1519
Merge pull request #1601 from randomecho/docs-contribute

Replace call to Show Source button to view file on GitHub [ci skip]
Documentation: Remove passive voice [ci skip]
Fix check for disallowing filenames more than 200 characters long
Shorten existing filenames over 200 characters for Windows users
Closes #1519
Merge pull request #1603 from randomecho/1519-docs-warn-windows-devs-on-filenames

Documentation: Update developer setup instructions to improve readability, correct URLs [ci skip]
Documentation: Remove section for Windows developers re: path length

On Windows your path must be less than 56 characters long, not 16 (see update to #1519). Not short enough to warrant extra note.
[DB MIGRATION REQ'D] Verified follower insight
Closes #1585, closes #1604
Capture tweet favorites count
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