Improve the description of epoch end

[jancionear]

The previous description of when an epoch ends was a bit confusing. It mentioned that two conditions are needed for a block to be the last block of an epoch, but in reality only one of them matters, as one condition is implied by the other. This makes the descripion harder to parse - why are there two conditions, if only one of them matters? It can lead to some confusion.

Let's improve the description to make it clearer and easier to understand. A diagram is also added for clarity.

Refs: zulip discussion

[jancionear]

It looks like the existing version of Epoch.md didn't pass the linter checks in the CI, so I fixed it.

The markdown-link-check fails because some links in unrelated files are throwing 404, I won't fix them as it's unrelated to this PR.

[frol]

@near/wg-protocol Could you, please, review the changes?

[jancionear]

@near/wg-protocol Could you, please, review the changes?

I'd appreciate a review, the PR is over a month old.

[jancionear]

Github says:

Code owner review required. Waiting on code owner review from near/nep-moderators

I guess it also needs a review from @near/nep-moderators before merging.