fix(content): update CryptoEcon Section content

[yiannisbot]

This PR is adding new description to the CryptoEconomics section of the spec. The description includes a table with the basic parameters, as well as justification for the design principles of Filecoin's cryptoeconomic model.

TODOs:

• Link to the minting model and token allocation when the corresponding PRs get merged.
• There will soon be a separate section that will summarise the parameters of the protocol. Once that section exists, the table included here will likely need to be excluded from here and replace with a pointer to that section, in order to have a single source of truth.

[yiannisbot]

The latest commit (6c73b4c) adds a subsection on Verified Clients as part of the CryptoEconomics section. However, given that the term is used earlier in the specification, I have included the same text in an update of the Intro section, currently being worked on in #1168. Given the central role of the term, I don't mind the redundancy, but I also don't mind excluding it in either of the two sections.

[yiannisbot]

Two changes in the latest commits:

1. it has been decided that the Verified Clients discussion is moved to a separate section of its own.
2. the cryptoecon parameters have been updated to reflect recommendations on latest values. @zixuanzh please indicate the correct value for the "Sector Termination Fee".

[zixuanzh]

LGTM