Updates Politeia docs

[s-ben]

Now that we've had Pi running for a while (and have updated copy in the Pi modal), it's time to updated and flesh out the Pi documentation on dcrdocs. This PR:

• Updates the proposal steps wording to match updated copy in Pi modal ( Closes Create 'Proposal Lifecycle' page #753 )
• Creates new page for example proposals ( Closes Create 'Proposal Example' page #752 )
• Creates new /Politeia submenu to house Pi docs (we have 3 pages now) ( Closes Create /Politeia submenu #754 )
• Creates new proposal guidance page, repurposing Pi rant about bad proposals
• General editing for flow

[xaur]

+1 for proposal guidelines page, it will be possible to link it compared to current copy buried in some .js file on github.

[xaur]

Created decred/politeiagui#947 to extract the .md files with the docs.

P.S. Such meta planning and discussion shall better happen in an issue, while pull requests are used for concrete implementation steps and link to the meta issue.

[s-ben]

@lukebp, nice suggestions. You may want to double check my rewordings for accuracy, but I think we're almost there. Also, while I have your eyeballs, can you check my attempted definition of Signature and Receipt for the comments.journal file in the pi data page? Issue: #697

@xaur, left a comment on that politeiagui Issue regarding the modal copy,
decred/politeiagui#947

[jholdstock]

Added a few in-line comments, but also one general comment.

This PR adds some new hyperlinks pointing to internal docs pages, and the link text is not capitalised. I think we might want to capitalise them so the formatting of the link text matches the formatting of the page title. I gathered a list as I was going through the PR:

politeia.md
	[introduction to Decred governance]
	[proposal guidelines]
	[example proposals]
	[navigating politeia data]

example-proposals.md
	[proposal guidelines]

proposal-guidelines.md
	[example proposals]

Thoughts about that?

[lukebp]

@s-ben the text that you updated to address my issues LGTM.

[s-ben]

@jholdstock , good catches. Have implemented your suggestions.

[jholdstock]

Two comments outstanding above. Otherwise everything is looking good to me.

[s-ben]

@jholdstock comments addressed. PR moon?

[RichardRed0x]

Looks good to me generally, I like the tips for creating a successful proposal.
I see the marketing example has ugly horizontal scroll bars, I recall trying to deal with something similar for the modal window display of that. Doesn't bother me too much personally.

[s-ben]

@RichardRed0x yeah @jholdstock was noticing the ugly too (his comment below for reference). I think it's the best we can do for now, but there's an open issue to address it.

I don't think we should be putting the proposal example text in codeblocks on this page. They aren't ideal for prose as they use the monospace font and they don't format nicely on small screens/windows.

That being said, we don't have many alternatives. The "correct" approach is probably to use blockquotes, but this would make the text green as a result of #566. I just tried this out and it doesn't look right.

Lets keep the codeblocks for now and I will open a new issue for this: #759