…ian (xabbuh) and Stof

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes#4611).Discussion----------Adding a guide about upgrading| Q             | A| ------------- | ---| Doc fix?      | no| New docs?     | yes| Applies to    | 2.6+| Fixed tickets |#4172Hi guys!First, the plan is to merge this into 2.6, then backport it to 2.3 (just remove the 2.5 and 2.6 sections about upgrading).I put this together fairly quickly, and I would really like comments. I've been in front of the computer for awhile, so I'm not convinced this is quite as smooth yet as I'd like it to be!Overall, I want to fix the issue of "How to I upgrade". We'll say - just go to this page, and it'll walk you through the steps, and even reassure you that you don't need to make any changes to your code (or, you only need to make these very specific changes).Thanks!Commits-------9701e46 Adding missing index/map documentsca1e7df [#4611] A few more tweaks and fixese6b3f13 Basically copying a section about upgrading other libraries down into the minor version section8195a66 [#4611] Removing a few entries I meant to remove before92b4310 [#4611] Making many tweaks thanks to guys like Javier, Wouter, Christian (xabbuh) and Stoff86b9c9 Adding a guide about upgrading

* 2.3: (33 commits)  Fixing bad reference  [#4743] parts -> part - subjective, but this sounds slightly better to me  Update web_server_configuration.rst  Update web_server_configuration.rst  Update web_server_configuration.rst  Add exception to console exception log  [#4454] Re-wording section, but it may not actually be accurate  More concrete explanation of validation groups  Adding missing index/map documents  [#4611] A few more tweaks and fixes  Basically copying a section about upgrading other libraries down into the minor version section  [#4611] Removing a few entries I meant to remove before  [#4611] Making many tweaks thanks to guys like Javier, Wouter, Christian (xabbuh) and Stof  Adding a guide about upgrading  Fix typo and remove redundant sentence  [Reference] Add default_locale config description  Clarify tip for creating a new AppBundle  Update forms.rst  Rewroded some contents and fixed some lists of elements  MAde some tweaks suggested by Wouter  ...Conflicts:book/http_cache.rst

* 2.5: (35 commits)  Fixing bad reference  [#4743] parts -> part - subjective, but this sounds slightly better to me  Update web_server_configuration.rst  Update web_server_configuration.rst  Update web_server_configuration.rst  Add exception to console exception log  [#4454] Re-wording section, but it may not actually be accurate  More concrete explanation of validation groups  Adding missing index/map documents  [#4611] A few more tweaks and fixes  Basically copying a section about upgrading other libraries down into the minor version section  [#4611] Removing a few entries I meant to remove before  [#4611] Making many tweaks thanks to guys like Javier, Wouter, Christian (xabbuh) and Stof  Adding a guide about upgrading  Fix typo and remove redundant sentence  document the `2.5` validation options  [Reference] Add default_locale config description  Clarify tip for creating a new AppBundle  Update forms.rst  Rewroded some contents and fixed some lists of elements  ...Conflicts:book/security.rst

* 2.7: (36 commits)  Fixing bad reference  [#4743] parts -> part - subjective, but this sounds slightly better to me  Update web_server_configuration.rst  Update web_server_configuration.rst  Update web_server_configuration.rst  Add exception to console exception log  [#4454] Re-wording section, but it may not actually be accurate  More concrete explanation of validation groups  Adding missing index/map documents  [#4611] A few more tweaks and fixes  Basically copying a section about upgrading other libraries down into the minor version section  [#4611] Removing a few entries I meant to remove before  [#4611] Making many tweaks thanks to guys like Javier, Wouter, Christian (xabbuh) and Stof  Adding a guide about upgrading  Fix typo and remove redundant sentence  Formatting fix  document the `2.5` validation options  [Reference] Add default_locale config description  Clarify tip for creating a new AppBundle  Update forms.rst  ...