Make a small grammatical adjustment

The original form of the phrase - «The other option is to specifically
checking if a session has expired after the session is started.» - is
not entirely grammatically correct, as the infinitive form of «to check»
shall normally be used, or the gerund form, which is the form used in
this commit.
[Cookbook][External Parameters] Enhance description of environment variables
Updating library/bundle installation docs to use the new composer require (no version) functionality
Fixing bad link
Correct a typo.

Removed uncorrect "load use" phrase part, and replaced it with "you can
use", which I guess conveys the original meaning of the phrase better.
first import of the "Official Best Practices" book
Fixed some technical problems as suggested by Wouter
Added an index directive to the first chapter
updated fabpot/sphinx-php module reference
added the missing "best-practice" directive import in Sphinx configuration
feature #4269 [Cookbook][External Parameters] Enhance content (bicpi)

This PR was merged into the 2.3 branch.

Discussion
----------

[Cookbook][External Parameters] Enhance content

Enhance description of environment variables used as container parameters

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Commits
-------

93684e2 [Cookbook][External Parameters] Enhance description of environment variables
minor #4276 [Components][HttpFoundation] Make a small grammatical adjustment (fabschurt)

This PR was merged into the 2.3 branch.

Discussion
----------

[Components][HttpFoundation] Make a small grammatical adjustment

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

The original form of the phrase - «The other option is to specifically
checking if a session has expired after the session is started.» - is
not entirely grammatically correct, as the infinitive form of «to check»
shall normally be used, or the gerund form, which is the form used in
this commit.

Commits
-------

3ad13a1 Make a small grammatical adjustment
[#4276] Making language correction we agreed on
minor #4325 [Components][Form] Correct a typo (fabschurt)

This PR was merged into the 2.3 branch.

Discussion
----------

[Components][Form] Correct a typo

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Removed uncorrect "load use" phrase part, and replaced it with "you can
use", which I guess conveys the original meaning of the phrase better.

Commits
-------

3d3388d Correct a typo.
Renamed "official best practices" to "best practices"
Fixed some typos reported by @henrikbjorn
Improve cookbook entry for error pages in 2.3~
feature #4294 Improve cookbook entry for error pages in 2.3~ (mpdude)

This PR was merged into the 2.3 branch.

Discussion
----------

Improve cookbook entry for error pages in 2.3~

This PR cherry-picks the improvements from #4293 that apply to 2.3 as well (almost all!).

It also addresses some of the considerations in #4181.

Commits
-------

f74b6f2 Improve cookbook entry for error pages in 2.3~
[#4290] Re-adding version back
Re-adding one more version