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.

Debian/Ubuntu's stock `~/.bashrc` prepends `~/.local/bin` to `PATH` towards its end if it exists.
Executables for per-user modules for system Python are installed into `~/.local/bin` --
so need to prepend `shims` to `PATH` later that that.

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

Change `done` to `fi` since otherwise Bash emits an unexpected token error:

```
bash: /home/foo/.bashrc: line 120: syntax error near unexpected token `done'
bash: /home/foo/.bashrc: line 120: `if command -v pyenv >/dev/null; then eval "$(pyenv init -)"; done'
```

Indirect ones with errata have proven to confuse users.
Fixes #1947, #1948

E.g. "*-latest"
May need splitting into project-specific ones if there emerge projects with different check needed

Closes https://github.com/pyenv/pyenv/issues/1559

* Update install instructions for Bash and Zsh

* Synchronize README.md with `pyenv init`
* Add a ~/.bash_profile note
* Concatenate shims activation into installation for brevity
  (Pyenv can't be used meaningfully without shims anyway)
  Otherwise, we'd need to duplicate all the ~/.profile shenanigans in both sections
* Update based on feedback
* Proofread

PATH manipulation should be done in ~/.profile rather than ~/.*rc since .rc can be sourced multiple times

Any whitespace doesn't work, need version per line.

Sometimes it is convenient to be able to temporarily disable something
in a version-file. Because these files often aren't necessarily tracked
in a SCM, especially when working with virtualenvs, the SCM diffs won't
help with showing removed lines which are currently the only way to
disable something.

The casing for `*_proxy` is currently inconsistent. I'm guessing that's a typo?

Thanks for maintaining this awesome tool!

corrected fish shell command