Closes #3069

[Fig](https://fig.io) has been sunlit for new users, with access being removed in September

[no ci]

This PR corrects a small but important detail in the documentation regarding how the system searches for executable files. The original text incorrectly stated that the operating system performs this search. The corrected text clarifies that it is actually the shell that performs the search.

Fixes breakage in WSL with Pyenv-win installed in the host system;
doesn't affect other supported installation scenarios.

Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

Recent feedback showed that it's not visible enough

Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

There was a space at the beginning of the `git clone` command

* Document listing available versions in the tutorial (#1677)
* Reflect 3.7.15's extended support
* Document using development head with Homebrew
* Structurize the Upgrading section
* Fix Pyenv-latest documentation

Added in https://github.com/pyenv/pyenv/pull/2463

The original instructions were added 2 years ago in 0f2d659732. Since then, [Fish released](https://github.com/fish-shell/fish-shell/releases/tag/3.2.0) [a more declarative instruction for this operation](https://fishshell.com/docs/current/cmds/fish_add_path.html), which is now the recommended way of manipulating `PATH` in Fish.

We're going to eliminate the need to mandatorily use `pyenv init --path`.
We can't delete it yet for backward compatibility.
Besides, there's one other use case for it: to enable shims but without
shell integration, e.g. for noninteractive shells.

To be a full-fledged replacement for `pyenv init -` however,
it needs to do rehashing.

Now the setup is to add to both rc and profile:
1) set PYENV_ROOT
   (can do it unconditionally -- since if you change it,
   you need to update all places anyway since any of them can be run first)
2) Add `pyenv` to PATH if not already there
3) eval "$(pyenv init -)"

Not a breaking change, old setup will continue to work.

Minor changes to make the docs better.

Closes #106

In GNU `sed`, the `-iEe` argument is equivalent to `--in-place=Ee`, which would create `~/.profileEe` as backup of `~/.profile` if the command executed successfully. However, because the `e` is no longer being processed as an expression argument, `sed` does not correctly join the expressions and exits with `sed: -e expression #2, char 10: unexpected }`.

The intent is to use extended regex, perform the changes in-place, and use a series of expressions, so `-Ei -e` is used instead.